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  • Boolean Operators | Quick Guide, Examples & Tips

Boolean Operators | Quick Guide, Examples & Tips

Published on May 2, 2022 by Eoghan Ryan . Revised on May 31, 2023.

Boolean operators are words and symbols, such as AND or NOT, that let you expand or narrow your search parameters when using a database or search engine. When you search using these operators, it is known as a Boolean search.

You can use Boolean operators such as AND , OR , and NOT alongside keywords to create a Boolean string that will refine your search to find the most relevant results and sources .

Boolean operators

Table of contents

How to use boolean operators, proximity operators, other interesting articles, frequently asked questions about boolean operators.

Knowing how to use Boolean operators effectively can save you a lot of time and help you to find useful sources, determine the relevance of your research topic, and create strong research questions . It’s also very helpful when you’re working on a literature review or systematic review .

AND will provide search results containing both or all of your keywords.

Use this when you want results that contain two or more specific keywords.

OR will provide search results containing at least one of your keywords.

Use this when you want results that contain at least one (though not necessarily both) of your chosen keywords.

NOT will provide search results containing the first of your keywords but not the second . Make sure to put your keywords in the correct order when using NOT, as the search results provided will exclude the latter keyword.

Use this when you want results that contain one specific keyword but not another.

Parentheses: ()

Parentheses allow you to group together keywords and control the order in which the terms will be searched, just like in a mathematical statement. Keywords and Boolean operators within parentheses will be searched first , followed by keywords outside parentheses.

For example, the combination shown below will provide results that are initially filtered to contain at least one of “id” or “ego,” and then further refined to also include “developmental.”

You can also use more than one pair of parentheses. The search will filter results based on the innermost parenthetical keywords first, followed by the keywords in outer parentheses, and finally the keyword(s) outside parentheses.

Quotation marks: “”

Quotation marks will provide search results containing the exact keyword(s) contained within.

Use this when you want results that contain a precise keyword or keywords.

Asterisk: *

An asterisk will provide search results containing variations of a root word.

Use this when you want results that contain the keyword that you entered or other words that begin with those letters.

Each database or search engine may use Boolean operators in slightly different ways. Therefore, it is important to check how Boolean operators work in the particular database or search engine you’re using.

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literature search boolean operators

Proximity operators are similar to Boolean operators. They let you filter results based on the proximity of one keyword to another.

Proximity operators include NEAR (N x ) , WITHIN (W x ) , and SENTENCE .

Note: Not all databases use proximity operators. Be sure to check the search operations of the database you are using.

If you want to know more about ChatGPT, AI tools , citation , and plagiarism , make sure to check out some of our other articles with explanations and examples.

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A Boolean search uses specific words and symbols known as Boolean operators (e.g., AND , OR ) alongside keywords to limit or expand search results. Boolean searches allow you to:

  • Prioritize keywords
  • Exclude keywords
  • Search exact keywords
  • Search variations of your keywords

Boolean operators are specific words and symbols that you can use to expand or narrow your search parameters when using a database or search engine.

The most common Boolean operators are AND , OR , NOT or AND NOT , quotation marks “” , parentheses () , and asterisks * .

Each Boolean operator has a unique function. For example, the Boolean operator AND will provide search results containing both/all of your keywords.

Proximity operators are specific words used alongside your chosen keywords that let you specify the proximity of one keyword in relation to another.

The most common proximity operators include NEAR ( N x ), WITHIN ( W x ), and SENTENCE .

Each proximity operator has a unique function. For example, N x allows you to find sources that contain the specified keywords within a set number of words ( x ) of each other.

When searching for sources in databases, think of specific keywords that are relevant to your topic , and consider variations on them or synonyms that might be relevant.

Once you have a clear idea of your research parameters and key terms, choose a database that is relevant to your research (e.g., Medline, JSTOR, Project MUSE).

Find out if the database has a “subject search” option. This can help to refine your search. Use Boolean operators to combine your keywords, exclude specific search terms, and search exact phrases to find the most relevant sources.

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  • Research Skills Blog

Research Basics: Using Boolean Operators to Build a Search

By Carol Hollier on 02-Dec-2020 10:45:00

Research Basics: Boolean Operators | IFIS Publishing

Boolean operators are the bones of any good literature search. These operators — or connecting words — tell a database how to combine search terms. Researchers use them to configure their searches to find more precise and relevant results. Knowing how to use them correctly makes all the difference between a frustrating and a satisfactory searching experience.

There are three main search-building Boolean Operators— AND , OR , and NOT .

AND is used to indicate that both terms are present in the results, or all terms if you connect more than two terms with AND.

Helpful to know : In some databases (like PubMed) you need to capitalize your Boolean operators, and in others (like Web of Science) you don’t, but it never hurts to capitalize them. So go ahead and do it!

Boolean operators | IFIS Publishing

Depending on where you are searching, your AND search can look like this:

Boolean operators | IFIS Publishing

All three configurations above are the same search.

Each additional term or concept that you add with AND will make your search more specific and targeted. Be aware that if you add a term with AND that is the “wrong” term, perhaps capturing an idea with a different word than researchers use, you risk missing relevant results.

Helpful to know about AND In some databases, if you don’t type a Boolean operator between words, the database will treat those words as though they have an AND between them. For example, any databases on the Web of Science platform do this. In others, such as any on the EBSCOhost platform, the database will treat the words as though they are either right next to each other (as a phrase) or close to each other. Exactly how close will depend on behind-the-scenes decisions made by your university library.   To check what’s happening in a database, run a search with two words with an AND, and then run it again without the AND. If the number of results is higher with the AND than without it, you’ll know that you need to type AND to avoid having the database treat your terms as a phrase or as adjacent terms. We will go into what this means in our upcoming blog post 'Research Basics: Phrases and Proximity'.   PubMed combines these approaches. It will pick up some words as phrases that it maps to MeSH headings, but words that don’t correspond to those headings will be treated as though they’ve been typed with an AND. See exactly how PubMed has interpreted your search by looking at the search details on the advanced search page.

While AND refines your search, OR broadens it. Use OR to capture concepts that might be represented with two or more different words. It can be a safeguard against missing results because you’ve used the wrong term, or because researchers have used different terms or different spellings in different parts of the world and over time to capture the concept.

In the diagram below, we use three related words that can all cover the same concept. When we connect each word with OR, we are telling the database that we are happy to have results that have fish in them, or results that have seafood in them, or results that have “sea food” in them. Results could also have both fish and seafood in them, or seafood and “sea food” , or “sea food” and fish , or all three terms. As long as one of the terms appears in the record, it will be included in the results list.

You’ll notice that, strictly speaking, fish and seafood aren’t synonyms, but they are related terms and might both be relevant to your search. This is something that you, the searcher, get to decide. Your decisions about what makes sense to connect with OR should be informed by the language that you see researchers using as you start scanning your results. You don’t need to restrict yourself to only two or three terms in an OR string. You could also, in this example, add in OR salmon OR trout OR sardines and on and on.

Boolean operators | IFIS Publishing

Depending on where you are searching, your OR search might look like this:

Boolean operators | IFIS Publishing

Notice that the string is typed in one search box and is contained within parentheses. It is generally a good idea to keep each concept that is captured by an OR string in one search box. The parentheses tell the database to keep these terms grouped together as a single concept. Some databases will automatically add in the parentheses around what you’ve typed in a single search box, and others won’t—but it never hurts to add them yourself.

Your OR search could also look like this:

Boolean operators | IFIS Publishing

NOT is a powerful, potentially dangerous Boolean operator that must be used with caution. The problem with NOT is that it removes results, and when they are gone you can’t see them so you don’t know if they were removed for the reason you thought they would be.

In the diagram below, all the results that will be returned will have the word potato in them unless they also had the word sweet in them.

Boolean operators | IFIS Publishing

In general, it’s safer to refine a search using AND rather than NOT, but there are times when NOT can help you check if your search is doing what you hope and think it is doing. For instance, you could run this search below to decide if you really want to keep “sweet potato*” together in a phrase, or if it would be better to connect the words as sweet AND potato* .  

Boolean operators | IFIS Publishing

Notice that to execute this search you need to put potato* AND sweet inside parentheses to indicate that that search needs to happen first before the phrase “sweet potato*” is removed. That will let you see all the records that have the words sweet and potato* in them unless the words are together as a phrase.

Combing Boolean operators

In the search above, AND was combined with NOT. For many searches you will want to combine different Boolean operators. Depending on what kind of search interface you are using, you can either build it all in one go, or you might need to do it step by step.

The way that you are most likely to build a search is to gather together a number of words to represent each concept, and then combine each concept to intersect with the others, like this:

Boolean operators | IFIS Publishing

Remember that a database will process, or group together, what you’ve put inside brackets first, so be sure to group together what should be together. Otherwise many will read your string left to right, although others will do something harder to deduce. To get a sense of how important the parentheses are, searching fusarium OR hydrophobin* AND gush* OR flow* AND beer* OR ale OR brew* OR malt* in FSTA on the EBSCOhost platform brings back 71,842 results, but searching (fusarium OR hydrophobin*) AND (gush* OR flow*) AND (beer* OR ale OR brew* OR malt) brings back 86, all of which actually do have each of the concepts in them.

Boolean operators can be a bit confusing, but with a bit of practice and care it is easy to become adept at building search strings that return results that are relevant to the research you are pursuing.

For more information about laying the foundation for strong search skills, we will soon be publishing the following blog posts:

Research Basics: Phrases and Proximity

Research Basics: Truncation.

Please check back soon or sign up for the Research Skills blog mailing list to hear when these are live.

You may also be interested in learning the difference between a systematic review and a systematic literature review , or  literature searching best practice .

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Effective Literature Searching: Boolean Operators

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Boolean Operators

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Linking your concepts correctly will help you find the most suitable results. Many databases, including PubMed employ "Boolean operators." Boolean operators are words that connect search terms together to broaden or narrow the results retrieved.

The three Boolean operators are AND, OR, and NOT.

  • AND narrows your search by limiting your results to items that contain both words connected with AND.
  • OR expands your search by including results that contain one word, the other word, or both words.
  • NOT narrows your search by limiting your results to items that contain the word you designate before NOT, but not the word after NOT.

Cecelia Vetter, CC BY-SA 4.0, via Wikimedia Commons

Attribution: Cecelia Vetter, CC BY-SA 4.0, via Wikimedia Commons

  • << Previous: Building a Search Strategy
  • Next: Identifying Keywords & Subject Headings >>
  • Last Updated: Dec 29, 2023 2:55 PM
  • URL: https://library.aah.org/guides/literaturesearching

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Literature Searching

  • Step 1: Defining the search
  • Step 2. Choosing the right database
  • Step 3: Constructing the search
  • Searching techniques: using Boolean
  • MeSH and CINAHL Headings
  • What are Primary and Secondary Sources of Information
  • Step 4: Conducting the search
  • Step 5: Accessing the search results
  • How to use databases videos
  • Advanced Literature Searching: what are the different types of literature reviews?
  • Advanced Searching: Systematic Reviews
  • Advanced Searching: help with developing your research question
  • Advanced Searching: critical appraisal
  • Screening and Selecting studies

What is Boolean? How to make your searching easier

A normal keyword search might be ok for quick and general search but you may find too many results to scroll through. Many of the results will be irrelevant too.

When searching it helps to know what you're looking for, what you're not looking for and having the skills to find it. Using Boolean terms can help. Boolean Search uses a combination of keywords  to organise and sift through your searches. 

Using Boolean techniques when searching can:

  • Give more accurate and relevant search results
  • You can develop a broad or narrow search terms
  • Helps you navigate through appropriate research and disregard unrelated research
  • You can apply as many Boolean terms in the same search

Use any or a combination of the techniques below to make searching easier for you.

Boolean techniques

  • Phrase Searching "....."
  • Truncation .....*
  • Advanced Boolean: Parenthesis/Nesting (......)
  • Advanced Boolean: Proximity NEAR, W3 ...

Using AND between keywords narrows your search results from just a keyword search

Using OR extends your search out which retrieves more results. Works very well with very similar keywords and means you're not missing out on any relevant research.

Use NOT to exclude results that you're interested in and reduce the number of records retrieved.

Phrase searching works really well on all databases. Works well for a phrase that can also work as separate keywords.

Truncation can extend your search out to ensure you're not missing out on any relevant records. Works with words with different endings. Go to last letter of word before ending changes and add * . Here's some examples:

You can use  parentheses/nesting  to build a search with a combination of Boolean Operators by using (...). The search inside the (...) is always done first so this can affect your search results. 

Some databases allow for searching for words within the same sentence or within a defined number of words within one another e.g.

  • << Previous: Step 3: Constructing the search
  • Next: MeSH and CINAHL Headings >>
  • Last Updated: Aug 18, 2023 9:59 AM
  • URL: https://libguides.dundee.ac.uk/literaturesearching

Searching with Boolean Operators

Jun 21, 2022 • knowledge, information.

Boolean logic defines logical relationships between terms in a search. The Boolean search operators are and , or and not . You can use these operators to create a very broad or very narrow search.

  • And combines search terms so that each search result contains all of the terms. For example, travel and Europe finds articles that contain both travel and Europe.
  • Or combines search terms so that each search result contains at least one of the terms. For example, college or university finds results that contain either college or university.
  • Not excludes terms so that each search result does not contain any of the terms that follow it. For example, television not cable finds results that contain television but not cable.
  • When executing a search, And takes precedence over Or .
  • When you search EBSCO Discovery Service, your library administrator may require Boolean Operators be capitalized (AND, OR, NOT).

The following table illustrates the operation of Boolean terms:

When a single Find field is displayed, you can enter search terms in the Find field, and combine with AND, OR, and NOT. (For example, Roosevelt NOT Franklin .)

Search screen with Boolean

When Guided-Style Find fields are displayed, you can enter search terms in each Find field, and select AND, OR, and NOT from the Boolean drop-down lists.

boolean drop-down menus in advanced search

With longer search strings, you can combine many terms in a search with the AND operator, which will narrow your search results. For example, heart AND lung AND bypass AND artery will provide a more focused search than heart AND lung OR bypass OR artery .

To make even better use of Boolean operators, you can enclose search terms and their operators in parentheses to specify the order in which they are interpreted. Information within parentheses is read first, and then information outside parentheses is read next. For example, (heart OR lung) AND bypass will return different results than heart OR lung AND bypass .

Using Booleans and Parentheses

To make even better use of Boolean operators, you can use parentheses to nest query terms within other query terms.

You can enclose search terms and their operators in parentheses to specify the order in which they are interpreted . Information within parentheses is read first , then information outside parentheses is read next . For example,

When you enter (mouse OR rat) AND trap , the search engine retrieves results containing the word mouse or the word rat together with the word trap in the fields searched by default. If there are nested parentheses, the search engine processes the innermost parenthetical expression first, then the next, and so on until the entire query has been interpreted. For example,

((mouse OR rat) AND trap) OR mousetrap

If the Boolean queries do not include parentheses, mouse OR rat AND trap NOT mousetrap , the search engine will follow an order of operations like in math where AND and NOT are considered multiplication and are processed first, followed by the OR clause which is addition. Effectively, the search engine processes mouse OR rat AND trap NOT mousetrap as though the query has parentheses in the following places: mouse OR ((rat AND trap) NOT mousetrap)

Using Booleans When Phrase Searching

When Boolean operators are contained within a phrase that is enclosed in quotation marks, the operator is treated as a stop word. When this is the case, any single word will be searched for in its place.

literature search boolean operators

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  • v.106(4); 2018 Oct

A systematic approach to searching: an efficient and complete method to develop literature searches

Associated data.

Creating search strategies for systematic reviews, finding the best balance between sensitivity and specificity, and translating search strategies between databases is challenging. Several methods describe standards for systematic search strategies, but a consistent approach for creating an exhaustive search strategy has not yet been fully described in enough detail to be fully replicable. The authors have established a method that describes step by step the process of developing a systematic search strategy as needed in the systematic review. This method describes how single-line search strategies can be prepared in a text document by typing search syntax (such as field codes, parentheses, and Boolean operators) before copying and pasting search terms (keywords and free-text synonyms) that are found in the thesaurus. To help ensure term completeness, we developed a novel optimization technique that is mainly based on comparing the results retrieved by thesaurus terms with those retrieved by the free-text search words to identify potentially relevant candidate search terms. Macros in Microsoft Word have been developed to convert syntaxes between databases and interfaces almost automatically. This method helps information specialists in developing librarian-mediated searches for systematic reviews as well as medical and health care practitioners who are searching for evidence to answer clinical questions. The described method can be used to create complex and comprehensive search strategies for different databases and interfaces, such as those that are needed when searching for relevant references for systematic reviews, and will assist both information specialists and practitioners when they are searching the biomedical literature.

INTRODUCTION

Librarians and information specialists are often involved in the process of preparing and completing systematic reviews (SRs), where one of their main tasks is to identify relevant references to include in the review [ 1 ]. Although several recommendations for the process of searching have been published [ 2 – 6 ], none describe the development of a systematic search strategy from start to finish.

Traditional methods of SR search strategy development and execution are highly time consuming, reportedly requiring up to 100 hours or more [ 7 , 8 ]. The authors wanted to develop systematic and exhaustive search strategies more efficiently, while preserving the high sensitivity that SR search strategies necessitate. In this article, we describe the method developed at Erasmus University Medical Center (MC) and demonstrate its use through an example search. The efficiency of the search method and outcome of 73 searches that have resulted in published reviews are described in a separate article [ 9 ].

As we aimed to describe the creation of systematic searches in full detail, the method starts at a basic level with the analysis of the research question and the creation of search terms. Readers who are new to SR searching are advised to follow all steps described. More experienced searchers can consider the basic steps to be existing knowledge that will already be part of their normal workflow, although step 4 probably differs from general practice. Experienced searchers will gain the most from reading about the novelties in the method as described in steps 10–13 and comparing the examples given in the supplementary appendix to their own practice.

CREATING A SYSTEMATIC SEARCH STRATEGY

Our methodology for planning and creating a multi-database search strategy consists of the following steps:

  • Determine a clear and focused question
  • Describe the articles that can answer the question
  • Decide which key concepts address the different elements of the question
  • Decide which elements should be used for the best results
  • Choose an appropriate database and interface to start with
  • Document the search process in a text document
  • Identify appropriate index terms in the thesaurus of the first database
  • Identify synonyms in the thesaurus
  • Add variations in search terms
  • Use database-appropriate syntax, with parentheses, Boolean operators, and field codes
  • Optimize the search
  • Evaluate the initial results
  • Check for errors
  • Translate to other databases
  • Test and reiterate

Each step in the process is reflected by an example search described in the supplementary appendix .

1. Determine a clear and focused question

A systematic search can best be applied to a well-defined and precise research or clinical question. Questions that are too broad or too vague cannot be answered easily in a systematic way and will generally result in an overwhelming number of search results. On the other hand, a question that is too specific will result into too few or even zero search results. Various papers describe this process in more detail [ 10 – 12 ].

2. Describe the articles that can answer the question

Although not all clinical or research questions can be answered in the literature, the next step is to presume that the answer can indeed be found in published studies. A good starting point for a search is hypothesizing what the research that can answer the question would look like. These hypothetical (when possible, combined with known) articles can be used as guidance for constructing the search strategy.

3. Decide which key concepts address the different elements of the question

Key concepts are the topics or components that the desired articles should address, such as diseases or conditions, actions, substances, settings, domains (e.g., therapy, diagnosis, etiology), or study types. Key concepts from the research question can be grouped to create elements in the search strategy.

Elements in a search strategy do not necessarily follow the patient, intervention, comparison, outcome (PICO) structure or any other related structure. Using the PICO or another similar framework as guidance can be helpful to consider, especially in the inclusion and exclusion review stage of the SR, but this is not necessary for good search strategy development [ 13 – 15 ]. Sometimes concepts from different parts of the PICO structure can be grouped together into one search element, such as when the desired outcome is frequently described in a certain study type.

4. Decide which elements should be used for the best results

Not all elements of a research question should necessarily be used in the search strategy. Some elements are less important than others or may unnecessarily complicate or restrict a search strategy. Adding an element to a search strategy increases the chance of missing relevant references. Therefore, the number of elements in a search strategy should remain as low as possible to optimize recall.

Using the schema in Figure 1 , elements can be ordered by their specificity and importance to determine the best search approach. Whether an element is more specific or more general can be measured objectively by the number of hits retrieved in a database when searching for a key term representing that element. Depending on the research question, certain elements are more important than others. If articles (hypothetically or known) exist that can answer the question but lack a certain element in their titles, abstracts, or keywords, that element is unimportant to the question. An element can also be unimportant because of expected bias or an overlap with another element.

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Object name is jmla-106-531-f001.jpg

Schema for determining the optimal order of elements

Bias in elements

The choice of elements in a search strategy can introduce bias through use of overly specific terminology or terms often associated with positive outcomes. For the question “does prolonged breastfeeding improve intelligence outcomes in children?,” searching specifically for the element of duration will introduce bias, as articles that find a positive effect of prolonged breastfeeding will be much more likely to mention time factors in their titles or abstracts.

Overlapping elements

Elements in a question sometimes overlap in their meaning. Sometimes certain therapies are interventions for one specific disease. The Lichtenstein technique, for example, is a repair method for inguinal hernias. There is no need to include an element of “inguinal hernias” to a search for the effectiveness of the Lichtenstein therapy. Likewise, sometimes certain diseases are only found in certain populations. Adding such an overlapping element could lead to missing relevant references.

The elements to use in a search strategy can be found in the plot of elements in Figure 1 , by following the top row from left to right. For this method, we recommend starting with the most important and specific elements. Then, continue with more general and important elements until the number of results is acceptable for screening. Determining how many results are acceptable for screening is often a matter of negotiation with the SR team.

5. Choose an appropriate database and interface to start with

Important factors for choosing databases to use are the coverage and the presence of a thesaurus. For medically oriented searches, the coverage and recall of Embase, which includes the MEDLINE database, are superior to those of MEDLINE [ 16 ]. Each of these two databases has its own thesaurus with its own unique definitions and structure. Because of the complexity of the Embase thesaurus, Emtree, which contains much more specific thesaurus terms than the MEDLINE Medical Subject Headings (MeSH) thesaurus, translation from Emtree to MeSH is easier than the other way around. Therefore, we recommend starting in Embase.

MEDLINE and Embase are available through many different vendors and interfaces. The choice of an interface and primary database is often determined by the searcher’s accessibility. For our method, an interface that allows searching with proximity operators is desirable, and full functionality of the thesaurus, including explosion of narrower terms, is crucial. We recommend developing a personal workflow that always starts with one specific database and interface.

6. Document the search process in a text document

We advise designing and creating the complete search strategies in a log document, instead of directly in the database itself, to register the steps taken and to make searches accountable and reproducible. The developed search strategies can be copied and pasted into the desired databases from the log document. This way, the searcher is in control of the whole process. Any change to the search strategy should be done in the log document, assuring that the search strategy in the log is always the most recent.

7. Identify appropriate index terms in the thesaurus of the first database

Searches should start by identifying appropriate thesaurus terms for the desired elements. The thesaurus of the database is searched for matching index terms for each key concept. We advise restricting the initial terms to the most important and most relevant terms. Later in the process, more general terms can be added in the optimization process, in which the effect on the number of hits, and thus the desirability of adding these terms, can be evaluated more easily.

Several factors can complicate the identification of thesaurus terms. Sometimes, one thesaurus term is found that exactly describes a specific element. In contrast, especially in more general elements, multiple thesaurus terms can be found to describe one element. If no relevant thesaurus terms have been found for an element, free-text terms can be used, and possible thesaurus terms found in the resulting references can be added later (step 11).

Sometimes, no distinct thesaurus term is available for a specific key concept that describes the concept in enough detail. In Emtree, one thesaurus term often combines two or more elements. The easiest solution for combining these terms for a sensitive search is to use such a thesaurus term in all elements where it is relevant. Examples are given in the supplementary appendix .

8. Identify synonyms in the thesaurus

Most thesauri offer a list of synonyms on their term details page (named Synonyms in Emtree and Entry Terms in MeSH). To create a sensitive search strategy for SRs, these terms need to be searched as free-text keywords in the title and abstract fields, in addition to searching their associated thesaurus terms.

The Emtree thesaurus contains more synonyms (300,000) than MeSH does (220,000) [ 17 ]. The difference in number of terms is even higher considering that many synonyms in MeSH are permuted terms (i.e., inversions of phrases using commas).

Thesaurus terms are ordered in a tree structure. When searching for a more general thesaurus term, the more specific (narrower) terms in the branches below that term will also be searched (this is frequently referred to as “exploding” a thesaurus term). However, to perform a sensitive search, all relevant variations of the narrower terms must be searched as free-text keywords in the title or abstract, in addition to relying on the exploded thesaurus term. Thus, all articles that describe a certain narrower topic in their titles and abstracts will already be retrieved before MeSH terms are added.

9. Add variations in search terms (e.g., truncation, spelling differences, abbreviations, opposites)

Truncation allows a searcher to search for words beginning with the same word stem. A search for therap* will, thus, retrieve therapy, therapies, therapeutic, and all other words starting with “therap.” Do not truncate a word stem that is too short. Also, limitations of interfaces should be taken into account, especially in PubMed, where the number of search term variations that can be found by truncation is limited to 600.

Databases contain references to articles using both standard British and American English spellings. Both need to be searched as free-text terms in the title and abstract. Alternatively, many interfaces offer a certain code to replace zero or one characters, allowing a search for “pediatric” or “paediatric” as “p?ediatric.” Table 1 provides a detailed description of the syntax for different interfaces.

Field codes in five most used interfaces for biomedical literature searching

Searching for abbreviations can identify extra, relevant references and retrieve more irrelevant ones. The search can be more focused by combining the abbreviation with an important word that is relevant to its meaning or by using the Boolean “NOT” to exclude frequently observed, clearly irrelevant results. We advise that searchers do not exclude all possible irrelevant meanings, as it is very time consuming to identify all the variations, it will result in unnecessarily complicated search strategies, and it may lead to erroneously narrowing the search and, thereby, reduce recall.

Searching partial abbreviations can be useful for retrieving relevant references. For example, it is very likely that an article would mention osteoarthritis (OA) early in the abstract, replacing all further occurrences of osteoarthritis with OA . Therefore, it may not contain the phrase “hip osteoarthritis” but only “hip oa.”

It is also important to search for the opposites of search terms to avoid bias. When searching for “disease recurrence,” articles about “disease free” may be relevant as well. When the desired outcome is survival , articles about mortality may be relevant.

10. Use database-appropriate syntax, with parentheses, Boolean operators, and field codes

Different interfaces require different syntaxes, the special set of rules and symbols unique to each database that define how a correctly constructed search operates. Common syntax components include the use of parentheses and Boolean operators such as “AND,” “OR,” and “NOT,” which are available in all major interfaces. An overview of different syntaxes for four major interfaces for bibliographic medical databases (PubMed, Ovid, EBSCOhost, Embase.com, and ProQuest) is shown in Table 1 .

Creating the appropriate syntax for each database, in combination with the selected terms as described in steps 7–9, can be challenging. Following the method outlined below simplifies the process:

  • Create single-line queries in a text document (not combining multiple record sets), which allows immediate checking of the relevance of retrieved references and efficient optimization.
  • Type the syntax (Boolean operators, parentheses, and field codes) before adding terms, which reduces the chance that errors are made in the syntax, especially in the number of parentheses.
  • Use predefined proximity structures including parentheses, such as (() ADJ3 ()) in Ovid, that can be reused in the query when necessary.
  • Use thesaurus terms separately from free-text terms of each element. Start an element with all thesaurus terms (using “OR”) and follow with the free-text terms. This allows the unique optimization methods as described in step 11.
  • When adding terms to an existing search strategy, pay close attention to the position of the cursor. Make sure to place it appropriately either in the thesaurus terms section, in the title/abstract section, or as an addition (broadening) to an existing proximity search.

The supplementary appendix explains the method of building a query in more detail, step by step for different interfaces: PubMed, Ovid, EBSCOhost, Embase.com, and ProQuest. This method results in a basic search strategy designed to retrieve some relevant references upon which a more thorough search strategy can be built with optimization such as described in step 11.

11. Optimize the search

The most important question when performing a systematic search is whether all (or most) potentially relevant articles have been retrieved by the search strategy. This is also the most difficult question to answer, since it is unknown which and how many articles are relevant. It is, therefore, wise first to broaden the initial search strategy, making the search more sensitive, and then check if new relevant articles are found by comparing the set results (i.e., search for Strategy #2 NOT Strategy #1 to see the unique results).

A search strategy should be tested for completeness. Therefore, it is necessary to identify extra, possibly relevant search terms and add them to the test search in an OR relationship with the already used search terms. A good place to start, and a well-known strategy, is scanning the top retrieved articles when sorted by relevance, looking for additional relevant synonyms that could be added to the search strategy.

We have developed a unique optimization method that has not been described before in the literature. This method often adds valuable extra terms to our search strategy and, therefore, extra, relevant references to our search results. Extra synonyms can be found in articles that have been assigned a certain set of thesaurus terms but that lack synonyms in the title and/or abstract that are already present in the current search strategy. Searching for thesaurus terms NOT free-text terms will help identify missed free-text terms in the title or abstract. Searching for free-text terms NOT thesaurus terms will help identify missed thesaurus terms. If this is done repeatedly for each element, leaving the rest of the query unchanged, this method will help add numerous relevant terms to the query. These steps are explained in detail for five different search platforms in the supplementary appendix .

12. Evaluate the initial results

The results should now contain relevant references. If the interface allows relevance ranking, use that in the evaluation. If you know some relevant references that should be included in the research, search for those references specifically; for example, combine a specific (first) author name with a page number and the publication year. Check whether those references are retrieved by the search. If the known relevant references are not retrieved by the search, adapt the search so that they are. If it is unclear which element should be adapted to retrieve a certain article, combine that article with each element separately.

Different outcomes are desired for different types of research questions. For instance, in the case of clinical question answering, the researcher will not be satisfied with many references that contain a lot of irrelevant references. A clinical search should be rather specific and is allowed to miss a relevant reference. In the case of an SR, the researchers do not want to miss any relevant reference and are willing to handle many irrelevant references to do so. The search for references to include in an SR should be very sensitive: no included reference should be missed. A search that is too specific or too sensitive for the intended goal can be adapted to become more sensitive or specific. Steps to increase sensitivity or specificity of a search strategy can be found in the supplementary appendix .

13. Check for errors

Errors might not be easily detected. Sometimes clues can be found in the number of results, either when the number of results is much higher or lower than expected or when many retrieved references are not relevant. However, the number expected is often unknown, and very sensitive search strategies will always retrieve many irrelevant articles. Each query should, therefore, be checked for errors.

One of the most frequently occurring errors is missing the Boolean operator “OR.” When no “OR” is added between two search terms, many interfaces automatically add an “AND,” which unintentionally reduces the number of results and likely misses relevant references. One good strategy to identify missing “OR”s is to go to the web page containing the full search strategy, as translated by the database, and using Ctrl-F search for “AND.” Check whether the occurrences of the “AND” operator are deliberate.

Ideally, search strategies should be checked by other information specialists [ 18 ]. The Peer Review of Electronic Search Strategies (PRESS) checklist offers good guidance for this process [ 4 ]. Apart from the syntax (especially Boolean operators and field codes) of the search strategy, it is wise to have the search terms checked by the clinician or researcher familiar with the topic. At Erasmus MC, researchers and clinicians are involved during the complete process of structuring and optimizing the search strategy. Each word is added after the combined decision of the searcher and the researcher, with the possibility of directly comparing results with and without the new term.

14. Translate to other databases

To retrieve as many relevant references as possible, one has to search multiple databases. Translation of complex and exhaustive queries between different databases can be very time consuming and cumbersome. The single-line search strategy approach detailed above allows quick translations using the find and replace method in Microsoft Word (<Ctrl-H>).

At Erasmus MC, macros based on the find-and-replace method in Microsoft Word have been developed for easy and fast translation between the most used databases for biomedical and health sciences questions. The schema that is followed for the translation between databases is shown in Figure 2 . Most databases simply follow the structure set by the Embase.com search strategy. The translation from Emtree terms to MeSH terms for MEDLINE in Ovid often identifies new terms that need to be added to the Embase.com search strategy before the translation to other databases.

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Schematic representation of translation between databases used at Erasmus University Medical Center

Dotted lines represent databases that are used in less than 80% of the searches.

Using five different macros, a thoroughly optimized query in Embase.com can be relatively quickly translated into eight major databases. Basic search strategies will be created to use in many, mostly smaller, databases, because such niche databases often do not have extensive thesauri or advanced syntax options. Also, there is not much need to use extensive syntax because the number of hits and, therefore, the amount of noise in these databases is generally low. In MEDLINE (Ovid), PsycINFO (Ovid), and CINAHL (EBSCOhost), the thesaurus terms must be adapted manually, as each database has its own custom thesaurus. These macros and instructions for their installation, use, and adaptation are available at bit.ly/databasemacros.

15. Test and reiterate

Ideally, exhaustive search strategies should retrieve all references that are covered in a specific database. For SR search strategies, checking searches for their recall is advised. This can be done after included references have been determined by the authors of the systematic review. If additional papers have been identified through other non-database methods (i.e., checking references in included studies), results that were not identified by the database searches should be examined. If these results were available in the databases but not located by the search strategy, the search strategy should be adapted to try to retrieve these results, as they may contain terms that were omitted in the original search strategies. This may enable the identification of additional relevant results.

A methodology for creating exhaustive search strategies has been created that describes all steps of the search process, starting with a question and resulting in thorough search strategies in multiple databases. Many of the steps described are not new, but together, they form a strong method creating high-quality, robust searches in a relatively short time frame.

Our methodology is intended to create thoroughness for literature searches. The optimization method, as described in step 11, will identify missed synonyms or thesaurus terms, unlike any other method that largely depends on predetermined keywords and synonyms. Using this method results in a much quicker search process, compared to traditional methods, especially because of the easier translation between databases and interfaces (step 13). The method is not a guarantee for speed, since speed depends on many factors, including experience. However, by following the steps and using the tools as described above, searchers can gain confidence first and increase speed through practice.

What is new?

This method encourages searchers to start their search development process using empty syntax first and later adding the thesaurus terms and free-text synonyms. We feel this helps the searcher to focus on the search terms, instead of on the structure of the search query. The optimization method in which new terms are found in the already retrieved articles is used in some other institutes as well but has to our knowledge not been described in the literature. The macros to translate search strategies between interfaces are unique in this method.

What is different compared to common practice?

Traditionally, librarians and information specialists have focused on creating complex, multi-line (also called line-by-line) search strategies, consisting of multiple record sets, and this method is frequently advised in the literature and handbooks [ 2 , 19 – 21 ]. Our method, instead, uses single-line searches, which is critical to its success. Single-line search strategies can be easily adapted by adding or dropping a term without having to recode numbers of record sets, which would be necessary in multi-line searches. They can easily be saved in a text document and repeated by copying and pasting for search updates. Single-line search strategies also allow easy translation to other syntaxes using find-and-replace technology to update field codes and other syntax elements or using macros (step 13).

When constructing a search strategy, the searcher might experience that certain parentheses in the syntax are unnecessary, such as parentheses around all search terms in the title/abstract portion, if there is only one such term, there are double parentheses in the proximity statement, or one of the word groups exists for only one word. One might be tempted to omit those parentheses for ease of reading and management. However, during the optimization process, the searcher is likely to find extra synonyms that might consist of one word. To add those terms to the first query (with reduced parentheses) requires adding extra parentheses (meticulously placing and counting them), whereas, in the latter search, it only requires proper placement of those terms.

Many search methods highly depend on the PICO framework. Research states that often PICO or PICOS is not suitable for every question [ 22 , 23 ]. There are other acronyms than PICO—such as sample, phenomenon of interest, design, evaluation, research type (SPIDER) [ 24 ]—but each is just a variant. In our method, the most important and specific elements of a question are being analyzed for building the best search strategy.

Though it is generally recommended that searchers search both MEDLINE and Embase, most use MEDLINE as the starting point. It is considered the gold standard for biomedical searching, partially due to historical reasons, since it was the first of its kind, and more so now that it is freely available via the PubMed interface. Our method can be used with any database as a starting point, but we use Embase instead of MEDLINE or another database for a number of reasons. First, Embase provides both unique content and the complete content of MEDLINE. Therefore, searching Embase will be, by definition, more complete than searching MEDLINE only. Second, the number of terms in Emtree (the Embase thesaurus) is three times as high as that of MeSH (the MEDLINE thesaurus). It is easier to find MeSH terms after all relevant Emtree terms have been identified than to start with MeSH and translate to Emtree.

At Erasmus MC, the researchers sit next to the information specialist during most of the search strategy design process. This way, the researchers can deliver immediate feedback on the relevance of proposed search terms and retrieved references. The search team then combines knowledge about databases with knowledge about the research topic, which is an important condition to create the highest quality searches.

Limitations of the method

One disadvantage of single-line searches compared to multi-line search strategies is that errors are harder to recognize. However, with the methods for optimization as described (step 11), errors are recognized easily because missed synonyms and spelling errors will be identified during the process. Also problematic is that more parentheses are needed, making it more difficult for the searcher and others to assess the logic of the search strategy. However, as parentheses and field codes are typed before the search terms are added (step 10), errors in parentheses can be prevented.

Our methodology works best if used in an interface that allows proximity searching. It is recommended that searchers with access to an interface with proximity searching capabilities select one of those as the initial database to develop and optimize the search strategy. Because the PubMed interface does not allow proximity searches, phrases or Boolean “AND” combinations are required. Phrase searching complicates the process and is more specific, with the higher risk of missing relevant articles, and using Boolean “AND” combinations increases sensitivity but at an often high loss of specificity. Due to some searchers’ lack of access to expensive databases or interfaces, the freely available PubMed interface may be necessary to use, though it should never be the sole database used for an SR [ 2 , 16 , 25 ]. A limitation of our method is that it works best with subscription-based and licensed resources.

Another limitation is the customization of the macros to a specific institution’s resources. The macros for the translation between different database interfaces only work between the interfaces as described. To mitigate this, we recommend using the find-and-replace functionality of text editors like Microsoft Word to ease the translation of syntaxes between other databases. Depending on one’s institutional resources, custom macros can be developed using similar methods.

Results of the method

Whether this method results in exhaustive searches where no important article is missed is difficult to determine, because the number of relevant articles is unknown for any topic. A comparison of several parameters of 73 published reviews that were based on a search developed with this method to 258 reviews that acknowledged information specialists from other Dutch academic hospitals shows that the performance of the searches following our method is comparable to those performed in other institutes but that the time needed to develop the search strategies was much shorter than the time reported for the other reviews [ 9 ].

CONCLUSIONS

With the described method, searchers can gain confidence in their search strategies by finding many relevant words and creating exhaustive search strategies quickly. The approach can be used when performing SR searches or for other purposes such as answering clinical questions, with different expectations of the search’s precision and recall. This method, with practice, provides a stepwise approach that facilitates the search strategy development process from question clarification to final iteration and beyond.

SUPPLEMENTAL FILE

Acknowledgments.

We highly appreciate the work that was done by our former colleague Louis Volkers, who in his twenty years as an information specialist in Erasmus MC laid the basis for our method. We thank Professor Oscar Franco for reviewing earlier drafts of this article.

Literature Searching

In this guide.

  • Introduction
  • Steps for searching the literature in PubMed
  • Step 1 - Formulate a search question
  • Step 2- Identify primary concepts and gather synonyms
  • Step 3 - Locate subject headings (MeSH)
  • Step 4 - Combine concepts using Boolean operators
  • Step 5 - Refine search terms and search in PubMed
  • Step 6 - Apply limits

Precision vs Sensitivity

Searches for systematic reviews aim to be as extensive as possible to capture all available studies relevant to the research question. Thus, it is important to strike a balance between striving for comprehensiveness (sensitivity) and maintaining  relevance (precision) when developing a search strategy. Precise or focused searches retrieve less irrelevant results but have a higher risk of missing targeted literature. Sensitive or broad searches retrieve most of the targeted literature, but also return a higher proportion of irrelevant results. Increasing the comprehensiveness or sensitivity of a search will reduce its precision and retrieval of relevant studies. 

The sensitivity and precision of a search strategy are determined by the search concepts and terms (see Step 2). The more concepts that are added to the strategy with AND, the more precise and fewer results. The more search terms or synonyms that are added to the strategy with OR, the more sensitive and more results. 

literature search boolean operators

Boolean Operators

Boolean operators (AND, OR) are used to combine concepts and search terms in a database. It focuses the search, particularly when the concept contains multiple search terms or synonyms. It also connects various pieces of information or concepts together. 

literature search boolean operators

"diabetes mellitus" AND "telemedicine"

literature search boolean operators

"telemedicine" OR "mobile health" OR "mhealth" or "telehealth" 

Putting it together

literature search boolean operators

Search Order

Databases follow commands you specific and return results based on those commands. Keep in mind, Boolean operators assign logical order and commands to which terms/concepts get searched first. PubMed executes search commands from left to right and adds parenthesis to each step (see Search #1 and #2). If no boolean operators are specified, PubMed assumes each term is combined with AND (i.e. implicit AND) (see Search #4 below). If you are using a combination of AND or OR operators in a search, enclose the terms that are OR-ed together in parentheses

"diabetes" AND "mobile health" OR "telemedicine" 

Your results will include articles that relate to 1) diabetes and mobile health, or 2) telemedicine (see Search #1)

"diabetes" AND ("mobile health" OR "telemedicine")

Your results will include articles that relate to 1) diabetes and mobile health, or 2) diabetes and telemedicine (see Search #3). 

literature search boolean operators

Tip: To see how PubMed interpreted your search, click "Advanced" under the search bar, and in your search history, click on the arrow icon in the "Details" column. 

From the example earlier:

("diabetes OR "type 1 diabetes" OR "type 2 diabetes" OR "diabetes mellitus") AND ("mobile health technology" OR "mobile health app" OR "telemedicine" OR "mhealth") AND ("in-person care" OR "in-person visits" OR "in-person encounters" OR "in-person visits") 

  • << Previous: Step 3 - Locate subject headings (MeSH)
  • Next: Step 5 - Refine search terms and search in PubMed >>
  • Last Updated: Jan 9, 2024 10:30 AM
  • URL: https://laneguides.stanford.edu/LitSearch
  • UNC Libraries
  • HSL Academic Process
  • Systematic Reviews
  • Step 3: Conduct Literature Searches

Systematic Reviews: Step 3: Conduct Literature Searches

Created by health science librarians.

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  • Step 1: Complete Pre-Review Tasks
  • Step 2: Develop a Protocol

About Step 3: Conduct Literature Searches

Partner with a librarian, systematic searching process, choose a few databases, search with controlled vocabulary and keywords, acknowledge outdated or offensive terminology, helpful tip - building your search, use nesting, boolean operators, and field tags, build your search, translate to other databases and other searching methods, document the search, updating your review.

  • Searching FAQs
  • Step 4: Manage Citations
  • Step 5: Screen Citations
  • Step 6: Assess Quality of Included Studies
  • Step 7: Extract Data from Included Studies
  • Step 8: Write the Review

  Check our FAQ's

   Email us

  Chat with us (during business hours)

   Call (919) 962-0800

   Make an appointment with a librarian

  Request a systematic or scoping review consultation

Search the FAQs

In Step 3, you will design a search strategy to find all of the articles related to your research question. You will:

  • Define the main concepts of your topic
  • Choose which databases you want to search
  • List terms to describe each concept
  • Add terms from controlled vocabulary like MeSH
  • Use field tags to tell the database where to search for terms
  • Combine terms and concepts with Boolean operators AND and OR
  • Translate your search strategy to match the format standards for each database
  • Save a copy of your search strategy and details about your search

There are many factors to think about when building a strong search strategy for systematic reviews. Librarians are available to provide support with this step of the process.

Click an item below to see how it applies to Step 3: Conduct Literature Searches.

Reporting your review with PRISMA

For PRISMA, there are specific items you will want to report from your search.  For this step, review the PRISMA-S checklist.

  • PRISMA-S for Searching
  • Specify all databases, registers, websites, organizations, reference lists, and other sources searched or consulted to identify studies. Specify the date when each source was last searched or consulted. Present the full search strategies for all databases, registers and websites, including any filters and limits used.
  • For information on how to document database searches and other search methods on your PRISMA flow diagram, visit our FAQs "How do I document database searches on my PRISMA flow diagram?" and "How do I document a grey literature search for my PRISMA flow diagram?"

Managing your review with Covidence

For this step of the review, in Covidence you can:

  • Document searches in Covidence review settings so all team members can view
  • Add keywords from your search to be highlighted in green or red while your team screens articles in your review settings

How a librarian can help with Step 3

When designing and conducting literature searches, a librarian can advise you on :

  • How to create a search strategy with Boolean operators, database-specific syntax, subject headings, and appropriate keywords 
  • How to apply previously published systematic review search strategies to your current search
  • How to test your search strategy's performance 
  • How to translate a search strategy from one database's preferred structure to another

The goal of a systematic retrieve is to find all results that are relevant to your topic. Because systematic review searches can be quite extensive and retrieve large numbers of results, an important aspect of systematic searching is limiting the number of irrelevant results that need to be screened. Librarians are experts trained in literature searching and systematic review methodology. Ask us a question or partner with a librarian to save time and improve the quality of your review. Our comparison chart detailing two tiers of partnership provides more information on how librarians can collaborate with and contribute to systematic review teams.

Magnifying glass looking at city lights

Search Process

  • Use controlled vocabulary, if applicable
  • Include synonyms/keyword terms
  • Choose databases, websites, and/or registries to search
  • Translate to other databases
  • Search using other methods (e.g. hand searching)
  • Validate and peer review the search

Databases can be multidisciplinary or subject specific. Choose the best databases for your research question. Databases index various journals, so in order to be comprehensive, it is important to search multiple databases when conducting a systematic review. Consider searching databases with more diverse or global coverage (i.e., Global Index Medicus) when appropriate. A list of frequently used databases is provided below. You can access UNC Libraries' full listing of databases on the HSL website (arranged alphabetically or by subject ).

Generally speaking, when literature searching, you are not searching the full-text article. Instead, you are searching certain citation data fields, like title, abstract, keyword, controlled vocabulary terms, and more. When developing a literature search, a good place to start is to identify searchable concepts of the research question, and then expand by adding other terms to describe those concepts. Read below for more information and examples on how to develop a literature search, as well as find tips and tricks for developing more comprehensive searches.

Identify search concepts and terms for each

Start by identifying the main concepts of your research question. If unsure, try using a question framework to help identify the main searchable concepts. PICO is one example of a question framework and is used specifically for clinical questions. If your research question doesn't fit into the PICO model well, view other examples of question frameworks and try another!

Click to view our example in PICO format

Question: for patients 65 years and older, does an influenza vaccine reduce the future risk of pneumonia, controlled vocabulary.

Controlled vocabulary is a set of terminology assigned to citations to describe the content of each reference. Searching with controlled vocabulary can improve the relevancy of search results. Many databases assign controlled vocabulary terms to citations, but their naming schema is often specific to each database. For example, the controlled vocabulary system searchable via PubMed is MeSH, or Medical Subject Headings. More information on searching MeSH can be found here .

Note: Controlled vocabulary may be outdated, and some databases allow users to submit requests to update terminology.

Click to view Controlled Vocabulary for our example PICO

As mentioned above, databases with controlled vocabulary often use their own unique system. A listing of controlled vocabulary systems by database is shown below.

Keyword Terms

Not all citations are indexed with controlled vocabulary terms, however, so it is important to combine controlled vocabulary searches with keyword, or text word, searches. 

Authors often write about the same topic in varied ways and it is important to add these terms to your search in order to capture most of the literature. For example, consider these elements when developing a list of keyword terms for each concept:

  • American versus British spelling
  • hyphenated terms
  • quality of life
  • satisfaction
  • vaccination
  • influenza vaccination

There are several resources to consider when searching for synonyms. Scan the results of preliminary searches to identify additional terms. Look for synonyms, word variations, and other possibilities in Wikipedia, other encyclopedias or dictionaries, and databases. For example, PubChem lists additional drug names and chemical compounds.

Click to display Controlled Vocabulary and Keywords for our example PICO

Combining controlled vocabulary and text words in PubMed would look like this:

"Influenza Vaccines"[Mesh] OR "influenza vaccine" OR "influenza vaccines" OR "flu vaccine" OR "flu vaccines" OR "flu shot" OR "flu shots" OR "influenza virus vaccine" OR "influenza virus vaccines"

Social and cultural norms have been rapidly changing around the world. This has led to changes in the vocabulary used, such as when describing people or populations. Library and research terminology changes more slowly, and therefore can be considered outdated, unacceptable, or overly clinical for use in conversation or writing.

For our example with people 65 years and older, APA Style Guidelines recommend that researchers use terms like “older adults” and “older persons” and forgo terms like “senior citizens” and “elderly” that connote stereotypes. While these are current recommendations, researchers will recognize that terms like “elderly” have previously been used in the literature. Therefore, removing these terms from the search strategy may result in missed relevant articles. 

Research teams need to discuss current and outdated terminology and decide which terms to include in the search to be as comprehensive as possible. The research team or a librarian can search for currently preferred terms in glossaries, dictionaries, published guidelines, and governmental or organizational websites. The University of Michigan Library provides suggested wording to use in the methods section when antiquated, non-standard, exclusionary, or potentially offensive terms are included in the search.

Check the methods sections or supplementary materials of published systematic reviews for search strategies to see what terminology they used. This can help inform your search strategy by using MeSH terms or keywords you may not have thought of. However, be aware that search strategies will differ in their comprehensiveness.

You can also run a preliminary search for your topic, sort the results by Relevance or Best Match, and skim through titles and abstracts to identify terminology from relevant articles that you should include in your search strategy.

Nesting is a term that describes organizing search terms inside parentheses. This is important because, just like their function in math, commands inside a set of parentheses occur first. Parentheses let the database know in which order terms should be combined. 

Always combine terms for a single concept inside a parentheses set. For example: 

( "Influenza Vaccines"[Mesh] OR "influenza vaccine" OR "influenza vaccines" OR "flu vaccine" OR "flu vaccines" OR "flu shot" OR "flu shots" OR "influenza virus vaccine" OR "influenza virus vaccines" )

Additionally, you may nest a subset of terms for a concept inside a larger parentheses set, as seen below. Pay careful attention to the number of parenthesis sets and ensure they are matched, meaning for every open parentheses you also have a closed one.

( "Influenza Vaccines"[Mesh] OR "influenza vaccine" OR "influenza vaccines" OR "flu vaccine" OR "flu vaccines" OR "flu shot" OR "flu shots" OR "influenza virus vaccine" OR "influenza virus vaccines" OR   (( flu OR influenza ) AND ( vaccine OR vaccines OR vaccination OR immunization )))

Boolean operators

Boolean operators are used to combine terms in literature searches. Searches are typically organized using the Boolean operators OR or AND. OR is used to combine search terms for the same concept (i.e., influenza vaccine). AND is used to combine different concepts (i.e., influenza vaccine AND older adults AND pneumonia). An example of how Boolean operators can affect search retrieval is shown below. Using AND to combine the three concepts will only retrieve results where all are present. Using OR to combine the concepts will retrieve results that use all separately or together. It is important to note that, generally speaking, when you are performing a literature search you are only searching the title, abstract, keywords and other citation data. You are not searching the full-text of the articles.

boolean venn diagram example

The last major element to consider when building systematic literature searches are field tags. Field tags tell the database exactly where to search. For example, you can use a field tag to tell a database to search for a term in just the title, the title and abstract, and more. Just like with controlled vocabulary, field tag commands are different for every database.

If you do not manually apply field tags to your search, most databases will automatically search in a set of citation data points. Databases may also overwrite your search with algorithms if you do not apply field tags. For systematic review searching, best practice is to apply field tags to each term for reproducibility.

For example:

("Influenza Vaccines"[Mesh] OR "influenza vaccine"[tw] OR "influenza vaccines"[tw] OR "flu vaccine"[tw] OR "flu vaccines"[tw] OR "flu shot"[tw] OR "flu shots"[tw] OR "influenza virus vaccine"[tw] OR "influenza virus vaccines"[tw] OR ((flu[tw] OR influenza[tw]) AND (vaccine[tw] OR vaccines[tw] OR vaccination[tw] OR immunization[tw])))

Click to view field tags for several health databases

For more information about how to use a variety of databases, check out our guides on searching.

  • Searching PubMed guide Guide to searching Medline via the PubMed database
  • Searching Embase guide Guide to searching Embase via embase.com
  • Searching Scopus guide Guide to searching Scopus via scopus.com
  • Searching EBSCO Databases guide Guide to searching CINAHL, PsycInfo, Global Health, & other databases via EBSCO

Combining search elements together

Organizational structure of literature searches is very important. Specifically, how terms are grouped (or nested) and combined with Boolean operators will drastically impact search results. These commands tell databases exactly how to combine terms together, and if done incorrectly or inefficiently, search results returned may be too broad or irrelevant.

For example, in PubMed:

(influenza OR flu) AND vaccine is a properly combined search and it produces around 50,000 results.

influenza OR flu AND vaccine is not properly combined.  Databases may read it as everything about influenza OR everything about (flu AND vaccine), which would produce more results than needed.

We recommend one or more of the following:

  • put all your synonyms together inside a set of parentheses, then put AND between the closing parenthesis of one set and the opening parenthesis of the next set
  • use a separate search box for each set of synonyms
  • run each set of synonyms as a separate search, and then combine all your searches
  • ask a librarian if your search produces too many or too few results

Click to view the proper way to combine MeSH terms and Keywords for our example PICO

Question: for patients 65 years and older, does an influenza vaccine reduce the future risk of pneumonia , translating search strategies to other databases.

Databases often use their own set of terminology and syntax. When searching multiple databases, you need to adjust the search slightly to retrieve comparable results. Our sections on Controlled Vocabulary and Field Tags have information on how to build searches in different databases.  Resources to help with this process are listed below.

  • Polyglot search A tool to translate a PubMed or Ovid search to other databases
  • Search Translation Resources (Cornell) A listing of resources for search translation from Cornell University
  • Advanced Searching Techniques (King's College London) A collection of advanced searching techniques from King's College London

Other searching methods

Hand searching.

Literature searches can be supplemented by hand searching. One of the most popular ways this is done with systematic reviews is by searching the reference list and citing articles of studies included in the review. Another method is manually browsing key journals in your field to make sure no relevant articles were missed. Other sources that may be considered for hand searching include: clinical trial registries, white papers and other reports, pharmaceutical or other corporate reports, conference proceedings, theses and dissertations, or professional association guidelines.

Searching grey literature

Grey literature typically refers to literature not published in a traditional manner and often not retrievable through large databases and other popular resources. Grey literature should be searched for inclusion in systematic reviews in order to reduce bias and increase thoroughness. There are several databases specific to grey literature that can be searched.

  • Open Grey Grey literature for Europe
  • OAIster A union catalog of millions of records representing open access resources from collections worldwide
  • Grey Matters: a practical tool for searching health-related grey literature (CADTH) From CADTH, the Canadian Agency for Drugs and Technologies in Health, Grey Matters is a practical tool for searching health-related grey literature. The MS Word document covers a grey literature checklist, including national and international health technology assessment (HTA) web sites, drug and device regulatory agencies, clinical trial registries, health economics resources, Canadian health prevalence or incidence databases, and drug formulary web sites.
  • Duke Medical Center Library: Searching for Grey Literature A good online compilation of resources by the Duke Medical Center Library.

Systematic review quality is highly dependent on the literature search(es) used to identify studies. To follow best practices for reporting search strategies, as well as increase reproducibility and transparency, document various elements of the literature search for your review. To make this process more clear, a statement and checklist for reporting literature searches has been developed and and can be found below.

  • PRISMA-S: Reporting Literature Searches in Systematic Reviews
  • Section 4.5 Cochrane Handbook - Documenting and reporting the search process

At a minimum, document and report certain elements, such as databases searched, including name (i.e., Scopus) and platform (i.e. Elsevier), websites, registries, and grey literature searched. In addition, this also may include citation searching and reaching out to experts in the field. Search strategies used in each database or source should be documented, along with any filters or limits, and dates searched. If a search has been updated or was built upon previous work, that should be noted as well. It is also helpful to document which search terms have been tested and decisions made for term inclusion or exclusion by the team. Last, any peer review process should be stated as well as the total number of records identified from each source and how deduplication was handled. 

If you have a librarian on your team who is creating and running the searches, they will handle the search documentation.

You can document search strategies in word processing software you are familiar with like Microsoft Word or Excel, or Google Docs or Sheets. A template, and separate example file, is provided below for convenience. 

  • Search Strategy Documentation Template
  • Search Strategy Documentation Example

*Some databases like PubMed are being continually updated with new technology and algorithms. This means that searches may retrieve different results than when originally run, even with the same filters, date limits, etc.

When you decide to update a systematic review search, there are two ways of identifying new articles:  

1. rerun the original search strategy without any changes. .

Rerun the original search strategy without making any changes.  Import the results into your citation manager, and remove all articles duplicated from the original set of search results.

2. Rerun the original search strategy and add an entry date filter.

Rerun the original search strategy and add a date filter for when the article was added to the database ( not the publication date).  An entry date filter will find any articles added to the results since you last ran the search, unlike a publication date filter, which would only find more recent articles.

Some examples of entry date filters for articles entered since December 31, 2021 are:

  • PubMed:   AND ("2021/12/31"[EDAT] : "3000"[EDAT])
  • Embase: AND [31-12-2021]/sd
  • CINAHL:   AND EM 20211231-20231231
  • PsycInfo: AND RD 20211231-20231231
  • Scopus:   AND LOAD-DATE AFT 20211231  

Your PRISMA flow diagram

For more information about updating the PRISMA flow diagram for your systematic review, see the information on filling out a PRISMA flow diagram for review updates on the Step 8: Write the Review page of the guide.

  • << Previous: Step 2: Develop a Protocol
  • Next: Step 4: Manage Citations >>
  • Last Updated: Feb 8, 2024 9:22 AM
  • URL: https://guides.lib.unc.edu/systematic-reviews

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Searching Solutions: Boolean Searching

  • Keywords vs Indexed
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Boolean Concepts

Boolean searching (named after George Boole) is used by most databases to focus searches. Examples of each term and search strategy are given below. A few tips to start:

  • Three Boolean operators are recognized by most databases: AND, OR, and NOT
  • The operators must be written in ALL CAPS (there may be exceptions, but this is good practice and helps you keep track of your search terms)
  • The default in most databases is to combine the terms with the AND operator ("Ilene health 2015" becomes "Ilene AND health AND 2015")
  • Use a NOT operator to exclude terms from your search (see Example 3 below)
  • Databases generally have an order in which they process Boolean terms, but that order may differ between databases; Use parentheses to change the processing order
  • Always check the parentheses after a search to be certain the database put them where you expected           osteoporosis AND ("Alzheimer’s disease OR "Parkinson’s disease") is not the same as           (osteoporosis AND "Alzheimer’s disease") OR "Parkinson’s disease" (see below,  4. Examples , for additional examples)
  • Use quotation marks for phrases (unless you are using a database with automatic term mapping - like PubMed)

Use AND to connect two or more concepts

  • Narrows your search
  • Retrieve less results
  • Results will contain both concepts

Example 1: classroom management AND student behavior

BooleanAND

Some databases use "+" instead of (or in addition to) AND.

Use NOT to exclude concepts from your search.

  • Tells the database to ignore results with one concept - even if those results contain other concepts used the search

Example 3: osteoporosis NOT arthritis

Boolean NOT

Some databases use "-"  or AND NOT instead of NOT.

Other search options

In addition to Boolean terms, search strategies can include other options including Proximity Indicators (e.g., NEAR, ADJ, and PRE), truncation or Wildcard symbols (?, and *), the hyphen (-), and specialized options (e.g. SAME or EXACT). See the Additional Search Options tab.

Use OR to connect two or more similar concepts (synonyms)

  • Broadens your search
  • Retrieve more results
  • Not all results will contain both concepts

Example 2: assessments OR evaluations

literature search boolean operators

4. Examples

Here are a number of examples in combining three concepts with Boolean terms.

  • engineer AND Latino AND arthritis = 7 (only articles that include all three concepts)
  • engineer OR Latino OR arthritis = 1+2+3+4+5+6+7 (any article that contains any of these concepts)
  • engineer AND Latino = 5+7
  • (engineer AND Latino) NOT arthritis = 5
  • Latino OR engineer = 1+5+2+4+6+7
  • (engineer OR Latino) NOT arthritis = 1+5+2
  • engineer OR (Latino NOT arthritis) = 1+5+2+4
  • Latino NOT arthritis = 5+2

Boolean Example

Additional Resources

  • Search Help & Tips USC Libraries Search (Summon), Users Guide
  • << Previous: Keywords vs Indexed
  • Next: Additional Search Options >>
  • Last Updated: Jan 19, 2024 4:48 PM
  • URL: https://libguides.usc.edu/searching

Refining Boolean queries to identify relevant studies for systematic review updates

Affiliation.

  • 1 Computer Science Department, University of Sheffield, Sheffield, United Kingdom.
  • PMID: 33067630
  • PMCID: PMC7750994
  • DOI: 10.1093/jamia/ocaa148

Objective: Systematic reviews are important in health care but are expensive to produce and maintain. The authors explore the use of automated transformations of Boolean queries to improve the identification of relevant studies for updates to systematic reviews.

Materials and methods: A set of query transformations, including operator substitution, query expansion, and query reduction, were used to iteratively modify the Boolean query used for the original systematic review. The most effective transformation at each stage is identified using information about the studies included and excluded from the original review. A dataset consisting of 22 systematic reviews was used for evaluation. Updated queries were evaluated using the included and excluded studies from the updated version of the review. Recall and precision were used as evaluation measures.

Results: The updated queries were more effective than the ones used for the original review, in terms of both precision and recall. The overall number of documents retrieved was reduced by more than half, while the number of relevant documents found increased by 10.3%.

Conclusions: Identification of relevant studies for updates to systematic reviews can be carried out more effectively by using information about the included and excluded studies from the original review to produce improved Boolean queries. These updated queries reduce the overall number of documents retrieved while also increasing the number of relevant documents identified, thereby representing a considerable reduction in effort required by systematic reviewers.

Keywords: lexical statistics; query reformulation; screening; systematic reviews; systematic reviews updates.

© The Author(s) 2020. Published by Oxford University Press on behalf of the American Medical Informatics Association. All rights reserved. For permissions, please email: [email protected].

Publication types

  • Research Support, Non-U.S. Gov't
  • Datasets as Topic
  • Information Storage and Retrieval / methods
  • Systematic Reviews as Topic* / methods

Conducting a Literature Review

  • Getting Started
  • Developing a Question
  • Searching the Literature
  • Identifying Peer-Reviewed Resources
  • Managing Results
  • Analyzing the Literature
  • Writing the Review

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Search Strategies

  • Boolean Operators

Once you have identified the key concepts of your research question (see "Developing a Question"), you can use those concepts to develop keywords for your search strategy. The following tips and techniques will help you design a precise and relevant search strategy.

Keywords are any words you might use to search the record of an article, book, or other material in library databases. The database searches through the metadata (such as title, authors, publication, abstract, etc.) to find resources that contain the word you searched, and may also search through the full text of the material.

Keywords are most successful when you're searching for the words that the authors use to describe the research topic, as most databases will search for those specific words within the record of the article. To increase your chance of returning relevant results, consider all of the words that  might  be used to describe the research you're trying to find, and try some of these out in sample searches to determine which words return the best results.

Search Tips - Keywords

  • Search for singular and plural terms together: (physician OR physicians)
  • Search for both the American and British spelling of words: (behavior OR behaviour)
  • Search for synonyms of terms together: (teenager OR adolescent)
  • Search for phrases inside of quotation marks: ("young adult")

Use Boolean operators to combine keywords for more precise search results. 

AND - If the term must be included in your search:

influenza AND vaccine

OR - If terms are interchangeable, i.e. synonyms. Place OR'd terms within parentheses:

(influenza OR flu) AND vaccine

NOT - If a term should not be included in your search. This Boolean operator is rarely necessary for literature reviews.

(influenza OR flu) AND vaccine NOT H1N1

Note how we've used parentheses in the examples above. Search strings like these are similar to mathematical equations, where you perform the actions within the parentheses before proceeding from left to right to run the search. For example, using the search [(influenza OR flu) AND vaccine] will find results that have a term relating to influenza/flu, as well as the term vaccine.

If we moved the parentheses, it would be a very different search. [influenza OR (flu AND vaccine)] will provide results that use the term influenza, as well as results that use both the terms flu and vaccine. This means you would get results having to do with influenza but perhaps nothing to do with vaccination. 

Here are a few examples of how this search would be different depending on the arrangement of booleans and keywords. The area highlighted in pink represents the search results that would be returned with this search.

literature search boolean operators

Truncation allows you to quickly include all variations of a word in your search. Use the root of the keyword and add an asterisk (*). For example:

nurs* = nurse, nurses, nursing, nursery

IMPORTANT: Notice that "nursery" is also retrieved in the above search. Truncation will save you from having to include a large number of synonyms, but it will also add a certain number of irrelevant results. You can limit this effect by using the NOT Boolean operator, i.e. NOT nursery.

Wild cards allow you to replace a letter in a keyword to retrieve all variations of the spelling. For example:

p?ediatric = pediatric, paediatric

Free-Text vs. Thesaurus Searching

While you can search any word as a keyword, databases also contain an official list of the terms they use to describe the subject of each article, called Subject Headings. You can look up Subject Headings in the thesaurus of the database, using the thesaurus's search box to pull up the recommended Subject Heading for a given keyword. When searching specifically for Subject Headings, the database will only search the Subject Headings field within the record of each article (ie, not the title, abstract, etc.). This is a much more targeted method of searching, and is an excellent addition to your search strategy. 

A strong search strategy will use both free-text (keyword) searching and thesaurus searching, to ensure that all relevant articles have been retrieved by the search. The lists below outline the strengths and weaknesses of both types of search strategies.

Free-Text Searching

  • Natural language words describing your topic
  • More flexible search strategy - can use any term in any combination
  • Database looks for keywords anywhere in the record - not necessarily connected together
  • May yield too many or too few results
  • May yield many irrelevant results

Thesaurus Searching

  • Pre-defined "controlled vocabulary" words used to describe the content of each item in a database
  • Less flexible search strategy - need to know the exact controlled vocabulary term
  • Database looks for subjects only in the subject heading or descriptor field, where the most relevant words appear
  • If too many results, you can use subheadings to focus on one aspect of a broader topic
  • Results are usually very relevant to the topic

MIT Libraries. Database Search Tips: Keywords vs. Subjects. https://libguides.mit.edu/c.php?g=175963&p=1160804

Each database has their own thesaurus. You will need to adapt your search strategy for each database to take advantage of their unique thesaurus.

PubMed uses MeSH terms (Medical Subject Headings). You can learn more about finding and using MeSH terms here:

  • The Basics of Medical Subject Headings (MeSH) in MEDLINE/PubMed: A Tutorial

CINAHL uses CINAHL Headings. You can learn more about finding and use these terms here:

  • Using CINAHL/MeSH Headings

In other databases, look for a link with the terms "headings", "subject headings", or "thesaurus" to find the appropriate thesaurus terms for your search.

Citation Searching

Citation searching is a search strategy that allows you to search either forward or backwards time through the literature based on an identified relevant article:

You can search forward in time by using databases that allow you to search for other articles that have cited the identified relevant article. (Web of Science and Google Scholar can do this automatically.)

  • Web of Science (Clarivate Analytics)
  • Google Scholar

You can search backward in time by reviewing the reference list of the identified relevant article for additional article citations.

For more information about how to perform citation searches, check out this guide from the University of Toledo Libraries:

  • How To: Cited Reference Searches in Web of Science Guide from the University of Toledo

Retrieving Materials

Select a database.

When searching for articles, it is best to use an appropriate subject database rather than the SearchIT catalog. Be sure to select your database from the Spokane Academic Library homepage to ensure that you will have access to full-text articles.

screenshot of Spokane Academic Library's frequently used databases

"Find It @ WSU" Button in PubMed

When you have found an article that you would like to read in its entirety, look for the "Find It @ WSU" Button. This button will take you to the article entry in the SearchIT catalog.

Here's what that looks like in PubMed.

screenshot showing the Find It @ WSU button on a PubMed article

"Find It @ WSU" Button in CINAHL

screenshot showing the Find It @ WSU button on a CINAHL article

"Find It @ WSU" Button in PsycINFO

screenshot showing the Find It @ WSU button on PsychINFO search results page

Accessing the Full-Text Article

After selecting the "Find It @ WSU" Button, you will be taken to the article entry in SearchIT. Select the link under the Access Options box to be directed to the full-text article. 

screenshot of Search It item highlighting sign in and access links

If an article is not available in the WSU Libraries collection, you can request the article through interlibrary loan by selecting the link under "Access Options".

See the Using Interlibrary Loan section for more information.

screenshot of Search It item's access options

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  • Last Updated: Jan 23, 2024 11:31 AM
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Literature searching.

  • What is Literature Searching?
  • Developing a Research Question
  • Translating a Research Question
  • Boolean Operators
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literature search boolean operators

Below are our most commonly searched databases and their subject areas:

literature search boolean operators

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Searching CINAHL Plus Tutorial

Searching PubMed Tutorial

Using Medical Subject Headings (MeSH) in PubMed Tutorial

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  • Last Updated: Nov 30, 2023 12:35 PM
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Spartanburg Community College Library

  • Spartanburg Community College Library
  • SCC Research Guides

Search Strategies and Techniques

  • Boolean Operators (AND, OR, NOT)

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Boolean Operators

To search multiple keywords at once, you need to use connector terms, also called Boolean Operators . Using a Boolean Operator will tell the database to connect the terms together in your search.

There are three Boolean Operators:

  • AND: All keywords must appear in your results. AND will narrow down a search.
  • OR: Either of the keywords must appear in the results. OR will give you more results.
  • NOT: Excludes certain keywords from your results. NOT will narrow down a search.

Depending on the database you are using, there may be a place to choose which Boolean Operator you want to use, or you may have to type them in with your keywords.

Click on the below tabs to learn more and see examples of Boolean Operators.

AND is the most common of the Boolean Operators. AND will narrow down your search so you get less results, because it is telling the database that your search results must include every one of your search terms.

When you use AND to combine your keywords, keep in mind that while you will get results that have all of your keywords together, they might not necessarily be next to one another.

students AND community college

concussions AND football

textbooks AND student success

Searching Using AND - EBSCO 

Advanced Search in EBSCO for students AND community college with AND outlined in a red box with a red arrow pointing to it.

Searching Using AND - Gale

Advanced Search in Gale for students AND community college with AND outlined in a red box with a red arrow pointing to it.

The Boolean Operator OR broadens your search. Remember that in database searching, OR means MORE results. OR tells the database that you want results that mention one or both of your search terms.

OR is a helpful operator to use if you have a search term that has multiple meanings, like preschool OR nursery school. You will notice when you do your searching, that some authors might use the term "preschool" and others will use "nursery school" to mean the exact same thing. OR helps you make sure that you find the most possible articles about your topic. 

traumatic brain injury OR TBI

Artificial Intelligence OR AI

Salem Witch Trials OR Salem Witchcraft Trials

Searching Using OR - EBSCO

Advanced Search in EBSCO searching Artificial Intelligence OR AI with OR outlined in a red box with a red arrow pointing to it.

Searching Using OR - Gale

Advanced Search in Gale for Artificial intelligence OR AI with OR outlined in a red box with a red arrow pointing to it.

The Boolean Operator NOT helps narrow your search by excluding certain terms from your search. When using NOT, you are telling the database that you want information that is related to the first term, but not the second.

NOT is useful if one of your keywords has multiple meanings that keep giving you irrelevant results.

Hedy Lamarr NOT Hollywood

presidential elections NOT United States

dinosaurs NOT extinction

Searching Using NOT - EBSCO

Advanced Search in EBSCO for Hedy Lamarr NOT Hollywood with NOT in a red outlined box and a red arrow pointing to it.

Searching Using NOT - Gale

Advanced Search in Gale for Hedy Lamarr NOT Hollywood with NOT outlined in a red box with a red arrow pointing to it.

  • << Previous: Choosing Keywords
  • Next: Quotation Marks >>
  • Choosing the Right Database
  • Choosing Keywords
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  • Refining Search Results
  • Search Strategies and Techniques Workshop

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literature search boolean operators

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Boolean Operators

  • Watching now: Chapter 1: Introduction to Boolean Operators Start time: 00:00:00 End time: 00:02:13
  • Chapter 2: Using the AND, OR, and NOT Boolean Operators in a Literature Search Start time: 00:02:14 End time: 00:05:51
  • Chapter 3: Using Other Search Engine Functions for a Literature Search Start time: 00:05:52 End time: 00:07:35

Video Type: Tutorial

(2020). Boolean operators [Video]. Sage Research Methods. https:// doi. org/10.4135/9781529627374

"Boolean Operators." In Sage Video . : Muhammad Shakil Ahmad, 2020. Video, 00:07:35. https:// doi. org/10.4135/9781529627374.

, 2020. Boolean Operators , Sage Video. [Streaming Video] London: Sage Publications Ltd. Available at: <https:// doi. org/10.4135/9781529627374 & gt; [Accessed 21 Feb 2024].

Boolean Operators . Online video clip. SAGE Video. London: SAGE Publications, Ltd., 17 Nov 2022. doi: https:// doi. org/10.4135/9781529627374. 21 Feb 2024.

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Using Boolean operators in a systematic literature search, including the AND, OR, and NOT operators, and other search engine functions.

Chapter 1: Introduction to Boolean Operators

  • Start time: 00:00:00
  • End time: 00:02:13

Chapter 2: Using the AND, OR, and NOT Boolean Operators in a Literature Search

  • Start time: 00:02:14
  • End time: 00:05:51

Chapter 3: Using Other Search Engine Functions for a Literature Search

  • Start time: 00:05:52
  • End time: 00:07:35
  • Product: Sage Research Methods Video: Qualitative and Mixed Methods
  • Type of Content: Tutorial
  • Title: Boolean Operators
  • Publisher: Muhammad Shakil Ahmad
  • Series: Systematic Literature Review : A Practical Guide
  • Publication year: 2020
  • Online pub date: November 17, 2022
  • Discipline: Sociology , History , Economics , Criminology and Criminal Justice , Nursing , Science , Education , Business and Management , Communication and Media Studies , Marketing , Engineering , Mathematics , Medicine , Dentistry , Psychology , Health , Social Policy and Public Policy , Social Work , Political Science and International Relations , Geography , Public Health , Anthropology , Technology , Counseling and Psychotherapy , Computer Science
  • Methods: Mixed methods , Systematic review , Boolean logic
  • Duration: 00:07:35
  • DOI: https:// doi. org/10.4135/9781529627374
  • Keywords: Boolean logic , literature reviews , literature searches , mixed methods research , qualitative research methods , Systematic reviews Show all Show less
  • Online ISBN: 9781529627374 Copyright: Copyright 2020, Muhammad Shakil Ahmad More information Less information

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Literature searches

  • Related content
  • Peer review
  • Sabina Dosani , specialist registrar
  • Maudsley rotation, London s.dosani{at}medix-uk.com

Here are my tips for getting the most out of literature searches. Think of yourself as a super sleuth, following leads to solve clinical or research questions. Aim to be systematic and rigorous, but keep an open mind and anticipate surprises.

Before defining search terms, write down your subject and any related questions, themes, synonyms, or MeSH terms. Ask yourself, “What phrases would I use if indexing this topic?” Jot down alternative spellings, like oesophagus and esophagus.

Supervision

Before launching into your search, make notes of your proposed search strategy and discuss it with your research supervisor or other senior colleague. They can often help you make important modifications or suggest key articles you can plunder for MeSH terms.

Befriend librarians. Let them know what you're looking for and convey some enthusiasm. As well as your local hospital library, contact librarians at your royal college, the BMA, and other organisations, like the Royal Society of Medicine. Many of these will conduct literature searches on your behalf and offer a postal service. Most will demonstrate searching various specialist databases, not just Medline and the Cochrane collection. If you haven't got one, ask for an Athens password. This gives online access to many papers.

Finders keepers

Electronic indexes allow you to focus searches by limiting results to, say, review articles, English language publications, year of publication, and to combine search terms by using Boolean operators like “and”. Librarians can show you other useful ruses, like truncation. This means using a symbol, usually an asterix, at the end of a word. For instance, child * searches for child, children, and childhood. Where there is more than one spelling, question marks simplify searches—f?etal retrieves both foetal and fetal.

Paper trail

There's nothing more frustrating than finding that elusive reference and not being able to retrieve it later. It sounds dull but keeping records of searches and anything you have retrieved can save a lot of time and stress in the weeks to come. Get your literature search right, and you'll have firm foundations for future research. ■

literature search boolean operators

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How to undertake a literature search

Introduction.

Undertaking a literature search can be a daunting prospect. By breaking the exercise down into smaller steps, you can make the process more manageable. The following ten steps will help you complete the task from identifying key concepts to choosing databases for your search and saving your results and search strategy. It discusses each of the steps in a little more detail with examples and suggestions of where to get help.

There are ten steps to undertaking a literature search which we'll take you through below:

🎬 - Indicates a video is available with more information.

Please click on the boxes below to get a bit more detail on each step.

First, write out your title and check that you understand all the terms. Look up the meaning of any you don’t understand. An online dictionary or medical encyclopaedia may help with this.

If your search is for a dissertation, you may need to choose your own research question. In this case, you will need to consider whether there is likely to be enough research on your topic. Alternatively, if your topic is too broad, you could be overwhelmed by the number of references.

One way of checking how much is written on your topic is to use Library Search. Most libraries offer a Library Search or discovery tool. It provides a quick search across all the library’s holdings. You can also limit your search by date or type of document. If you just need a few references to help you write an essay, Library Search may be helpful. It also gives quick access to full text items.

Next, you need to identify your key concepts. One way to do this is to look at your title and identify the most important words. Ignore words that tell you what to do with the information you find eg evaluate, assess, compare, as these are not generally used as search terms. In the example below, key concepts have been highlighted:

Evaluate the effectiveness of a mindfulness intervention on the health-related quality of life of rheumatoid arthritis patients

Another way to do this is to break down your title using the PEO framework:

P = Population    E = Exposure    O = Outcome 

This works well where there is no comparison between two types of treatment or intervention.

In our example:

P = rheumatoid arthritis patients

E = mindfulness

O =  health related quality of life

Other question formats are available such as PICO or SPIDER

Tip: Not all search topics will include every element of PICO – some include fewer items.

Once you have identified the key concepts, it’s important to think of any other terms or phrases that might have a very similar meaning. Including such synonyms will make your search as thorough as possible. For example, if your topic is looking for articles on Staff attitudes , you might also use the terms:

  • Staff perceptions 
  • Staff opinions
  • Stereotyping
  • Labelling 

If the database you are using has a list of subject heading s , this may help you to find the most appropriate term for your subject. Some databases provide definitions for terms used in the database and may suggest related terms.

A comprehensive search will usually include both subject headings from databases and terms that you have thought of yourself.

Tip: Often your search term will be a phrase instead of a single word. To carry out phrase searches, use double quotes, for example “problem drinking”.

Once you have chosen your search terms, you need to think about the best databases for your topic. The databases you choose will depend on the search question and the libraries you have access to.

Tip: It’s well worth taking a few minutes to get to know the databases available on the Library webpages and what they cover.

The next step is to combine your search terms in such a way that you only retrieve the more relevant references for your search question. In order to do this you need to build a search strategy . This involves using Boolean operators such as AND , OR and NOT .

AND narrows the results of the search by ensuring that all the search terms are present in the results. 

OR broadens the results of the search by ensuring that any of the search terms are present in the results.

NOT limits the results by rejecting a particular search term. Be careful with NOT because it will exclude any results containing that search term regardless of whether other parts of the article might have been of interest.

OR will broaden your number of results while AND will produce fewer results.

Try using this  Search-plan-worksheet   to break your topic down into concepts. These can then be linked together when you run the search. You can also add synonyms within each concept box. The yellow limits box is a prompt to think about any limits you want to apply when searching. This leads us to Step 6.

Tip: Most databases will allow you to use a truncation sign (*) or wildcard (?) to pick up various different endings to words or alternative spellings.

For example:  alcohol* would pick up alcohol, alcoholic, alcoholism, etc

Sm?th would find Smith and Smyth

The next step is to think about any other restrictions you want to make to your results.

Common limiters found on databases include:

  • Peer reviewed articles
  • Research articles
  • Age group (adult, child, older person)
  • Document type

Not all databases allow all of the limiters above.

When writing a dissertation, primary research articles are normally required. Where the database allows you, try limiting to research articles only.

Non-research materials can also be useful as an overview of your topic; for example a literature review can give an analysis of what has already been written on a topic.

The video below includes a demonstration of how limits can be applied using the CINAHL database as an example:

CINAHL - advanced

Once you have identified all your search terms and any limits you want to apply, you are ready to run your search on the databases you have chosen. 

Once you have some search results, you can look through them and start to select those that look relevant to your literature search. It is likely you will reject some because they are not quite what you wanted but there will be others that can be marked for further attention.

The title of an article on its own may not tell you very much; read the abstract quite carefully to see if the article is relevant or not.

Tip: You can show more details for each record by clicking on the article title. On some databases, there may be an abstract for the article which you can open. 

If you find you are either generating more results than you can possibly look at or too few results to write about, be prepared to adjust your search terms and the way they are combined.

If you get too many results you could try: •limiting to just the most recent material •adding another term or concept and linking it using “AND” •limiting to a particular country or geographical area such as UK

If you get too few results, you might try: •expanding your date range •removing any geographical limits you have applied  •removing the least important term or concept

Tip: Be prepared to try other databases and keep searching until you feel confident you have found enough relevant material.

Once you have selected some articles that look relevant for your piece of work, you will need to save them so that your hard work is not wasted.

At the same time, you will want to save your search strategy . This is a record of the terms you searched, how you combined them, any limits you applied and how many results you found.

You will also need to choose a way to save your results. One way is to email the results to yourself and this can be done from all the databases .

Another way is to export your results to reference management software such as Zotero, RefWorks, EndNote or Mendeley. This software allows you to collect, organise and cite research. It is suitable for managing references over a long period of time. 

The RCN Library and Archive Service provides help with using Zotero . 

Tip: Keep a record of all the databases you use as you carry out your search. It is also a good idea to note where you found any references you subsequently use for your dissertation.

The final step is to obtain the full text of the articles identified in your search which you believe may be useful for your assignment. If you are lucky, many of these will be available electronically and you may just be able to follow a link to the full text.

Alternatively you can copy and paste your article title into the Library search box  and if it is available as full text, a hyperlink will be shown which will link you to the document.

If you are studying elsewhere and have access to a university or hospital library, they may subscribe to different journals to the RCN Library so it is worth exploring what they can offer. If your library does not have either an electronic copy or a physical copy, you may need to request the article by interlibrary loan .

Tip: It is also worth using Google or other browsers to check for the article title you require. Sometimes the article has been made freely available on the internet by the authors.

Boolean operators – words (AND, OR and NOT) which can be used to combine search terms in order to widen or limit the search results.

Database – this is an online collection of citations to journal articles which have been indexed to make retrieval easier. Some databases which also provide full text access to the articles.

Limits – these are options within a database which allow search results to be broken down further. Common limits are year(s) of publication, document type and language. MEDLINE and CINAHL allow age limits too.

Search Strategy – the list of search terms and limits used to retrieve relevant articles from a database in order to answer a search question.

Subject headings – terms that have been assigned to describe a concept that may have many alternative keywords. All these alternative keywords or terms are brought together under the umbrella of this single term. Most health-related databases use subject headings.

Additional information

If after following these steps, you still can’t find what you are looking for, remember that there is always help available at your library. The RCN Library and Archives Service offers a range of help materials via our Literature searching and training pages . These include: • Databases guides in electronic and printed formats • Video tutorials on how to search the databases • 1-1 training sessions pre-bookable via the RCN website face to face or via zoom

A reading list is also available on dissertation and essay support which provides suggestions for key resources, books and journal articles which may help. Click on the link below to access this list:

Dissertation and essay support reading list

Here are other resources you may also find helpful. You will find links to each resource below too:

  • Aveyard H (2019) Doing a literature review in health and social care: a practical guide . 4th edn. London: Open University Press.
  • Bettany-Satlikov J (2016) How to do a systematic literature review in nursing: a step-by-step guide . 2nd edn. Maidenhead: Open University Press.
  • Coughlan M and Cronin P (2016) Doing a literature review in nursing, health and social care . 2nd edn. Los Angeles: Sage.
  • De Brún C, Pearce-Smith N, Heneghan C, Perera R and Badenoch D (2014) Searching skills toolkit: finding the evidence . 2nd edn. Chichester: Wiley Blackwell / BMJ Books.
  • Hewitt-Taylor J (2017) The essential guide to doing a health and social care literature review . London: Routledge. 

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© 2024 Royal College of Nursing

  Al Harris Library

SWOSU

ENG 1213 Spring 2024:Smith: Search Strategies: Boolean

  • Peer Review
  • Search Strategies: Boolean
  • Search Strategies: Discovery & Databases
  • Research Tools
  • Ebook Collections
  • RefWorks: Citation Management Made Easy
  • Library Assignment
  • Annotated Bibliography

Video Example

https://www.youtube.com/watch?v=c9qypoyNsdE

Boolean Quiz

Boolean operators, boolean operators.

____________________________________________________________________________________________________________________

One of the basic techniques for searching is using the Boolean Operators. Boolean searching is a symbolic logic system that creates relationships between concepts and words.   

Use  BOOLEAN OPERATORS  to relate the terms in specific ways that will affect the results of a search. Boolean operators are terms that create relationships between concepts and words: 

Retrieves only articles that contain all the terms and will narrow the search. Examples:

Slaughterhouse Five AND meaning  Slaughterhouse Five AND literature satiric literature AND tragedy  satiric literature AND tragedy AND comedy  

Retrieves articles with any of the terms and will broaden the search. Examples:

Kurt Vonnegut OR modern authors  literature OR fiction  modern literature OR modern fiction or 21st century writers

Eliminates articles containing the third term even when the first two terms are present. This will narrow the search. Examples :

Kurt Vonnegut AND novels NOT Slaughterhouse Five Slaughterhouse Five AND novels NOT film reviews  film AND Kurt Vonnegut  NOT Slaughterhouse Five

literature search boolean operators

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  • Last Updated: Feb 20, 2024 2:53 PM
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IMAGES

  1. Advanced Search Strategies

    literature search boolean operators

  2. Search Operators

    literature search boolean operators

  3. The Ultimate Beginner's Guide to Boolean Search Term

    literature search boolean operators

  4. Search strategy design

    literature search boolean operators

  5. Boolean Search

    literature search boolean operators

  6. The basics of boolean search and 4 tips to get the best results

    literature search boolean operators

VIDEO

  1. Boolean Modifier Problem Solution

  2. Boolean Expression Reductions

  3. 006 BOOLEAN

  4. How to Use Boolean Search in Recruitment (Step-by-Step Examples)

  5. Boolean Search

  6. Intro to Databases

COMMENTS

  1. Boolean Operators

    Boolean operators are words and symbols, such as AND or NOT, that let you expand or narrow your search parameters when using a database or search engine. When you search using these operators, it is known as a Boolean search.

  2. Research Basics: Using Boolean Operators to Build a Search

    Boolean operators are the bones of any good literature search. These operators — or connecting words — tell a database how to combine search terms. Researchers use them to configure their searches to find more precise and relevant results.

  3. Boolean Operators

    The three Boolean operators are AND, OR, and NOT. AND narrows your search by limiting your results to items that contain both words connected with AND. OR expands your search by including results that contain one word, the other word, or both words.

  4. Literature search for research planning and identification of research

    Literature search is a key step in performing good authentic research. It helps in formulating a research question and planning the study. The available published data are enormous; therefore, choosing the appropriate articles relevant to your study in question is an art. ... Another strategy is combining keywords with Boolean operators. It is ...

  5. Searching techniques: using Boolean

    Phrase Searching "....." Truncation .....* Advanced Boolean: Parenthesis/Nesting (......) Advanced Boolean: Proximity NEAR, W3 ... Using AND between keywords narrows your search results from just a keyword search Part 3: Boolean Operators Watch on Last Updated: Aug 18, 2023 9:59 AM URL: https://libguides.dundee.ac.uk/literaturesearching Print Page

  6. Searching with Boolean Operators

    The Boolean search operators are and, or and not. You can use these operators to create a very broad or very narrow search. And combines search terms so that each search result contains all of the terms. For example, travel and Europe finds articles that contain both travel and Europe.

  7. A systematic approach to searching: an efficient and complete method to

    This method describes how single-line search strategies can be prepared in a text document by typing search syntax (such as field codes, parentheses, and Boolean operators) before copying and pasting search terms (keywords and free-text synonyms) that are found in the thesaurus.

  8. Step 4

    Boolean operators (AND, OR) are used to combine concepts and search terms in a database. It focuses the search, particularly when the concept contains multiple search terms or synonyms. It also connects various pieces of information or concepts together. Example: "diabetes mellitus" AND "telemedicine" Example:

  9. Systematic Reviews: Step 3: Conduct Literature Searches

    Boolean operators. Boolean operators are used to combine terms in literature searches. Searches are typically organized using the Boolean operators OR or AND. OR is used to combine search terms for the same concept (i.e., influenza vaccine). AND is used to combine different concepts (i.e., influenza vaccine AND older adults AND pneumonia).

  10. Subject Guides: Literature Searching: Boolean Operators

    Connecting Boolean Operators Together Sometimes, when you have a complicated research question, using multiple Boolean Operators in one search string can be helpful. In these instances, connecting different parts of the search string together using parentheses will help (it will resemble a simple math equation).

  11. How to carry out a literature search for a systematic review: a

    Literature reviews are conducted for the purpose of (a) locating information on a topic or identifying gaps in the literature for areas of future study, (b) synthesising conclusions in an area of ambiguity and (c) helping clinicians and researchers inform decision-making and practice guidelines.

  12. Searching Solutions: Boolean Searching

    Three Boolean operators are recognized by most databases: AND, OR, and NOT The operators must be written in ALL CAPS (there may be exceptions, but this is good practice and helps you keep track of your search terms) The default in most databases is to combine the terms with the AND operator ("Ilene health 2015" becomes "Ilene AND health AND 2015")

  13. Refining Boolean queries to identify relevant studies for ...

    Materials and methods: A set of query transformations, including operator substitution, query expansion, and query reduction, were used to iteratively modify the Boolean query used for the original systematic review.

  14. PDF Effective Literature Searching

    The three Boolean operators, AND, OR and NOT, are explained in Table 7.34 and Figure 7.1. 132 Organizing and Managing Your Research Table 7.3 The use of Boolean operators to refine a literature search. Operator Process Result OR Requires either or both terms Increases the number of to be present in the document, documents retrieved e.g. women ...

  15. LibGuides: The EBM Medical Literature Search: Boolean Operators

    The EBM Medical Literature Search What are Boolean Operators? Boolean Operators ( AND, OR, NOT) are commands used to make your search strategy more concise. In it's most simple use, library databases are searched effectively by linking two keywords with a Boolean Operator. This page covers AND, OR, NOT in both simple and advanced searches.

  16. Searching the Literature

    Boolean Operators Truncation Wild Cards Once you have identified the key concepts of your research question (see "Developing a Question"), you can use those concepts to develop keywords for your search strategy. The following tips and techniques will help you design a precise and relevant search strategy.

  17. Subject Guides: Literature Searching: Performing a Search

    Looking in multiple buckets offers more coverage. No single bucket will have all the information you need. Some databases also have subject-specific coverage, so your research question will also dictate the best databases to search. Below are our most commonly searched databases and their subject areas:

  18. Boolean Operators (AND, OR, NOT)

    Using a Boolean Operator will tell the database to connect the terms together in your search. There are three Boolean Operators: AND: All keywords must appear in your results. AND will narrow down a search. OR: Either of the keywords must appear in the results. OR will give you more results. NOT: Excludes certain keywords from your results.

  19. Boolean Operators

    Using Boolean operators in a systematic literature search, including the AND, OR, and NOT operators, and other search engine functions. ... Chapter 2: Using the AND, OR, and NOT Boolean Operators in a Literature Search icon angle down. Start time: 00:02:14; End time: 00:05:51; Chapter 3: Using Other Search Engine Functions for a Literature ...

  20. Search strategy formulation for systematic reviews: Issues, challenges

    Summary of problems with Boolean strategies Tools and approaches to augment search strategy development A number of tools and approaches have been proposed to either augment or replace the methods outlined above for the development of search strategies. Whilst they are gaining traction, they have yet to attract widespread use.

  21. Literature searches

    Electronic indexes allow you to focus searches by limiting results to, say, review articles, English language publications, year of publication, and to combine search terms by using Boolean operators like "and". Librarians can show you other useful ruses, like truncation. This means using a symbol, usually an asterix, at the end of a word.

  22. How to undertake a literature search

    Boolean operators - words (AND, OR and NOT) which can be used to combine search terms in order to widen or limit the search results. Database - this is an online collection of citations to journal articles which have been indexed to make retrieval easier. Some databases which also provide full text access to the articles.

  23. (PDF) Basics of Literature Searching using Boolean Logic

    Ammar Anwer University of Lahore Abstract This presentation was given in an online webinar. This presentation covers; 1. A basic Concept of Keywords & MeSH terms 2. Understanding Boolean Logic 3....

  24. ENG 1213 Spring 2024:Smith: Search Strategies: Boolean

    Boolean searching is a symbolic logic system that creates relationships between concepts and words. UseBOOLEAN OPERATORSto relate the terms in specific ways that will affect the results of a search. Boolean operators are terms that create relationships between concepts and words: Retrieves only articles that contain all the terms and will ...