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  • 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systematic searching process, choose a few databases, search with controlled vocabulary and keywords, acknowledge outdated or offensive terminology, helpful tip - building your search, use nesting, boolean operators, and field tags, build your search, translate to other databases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

   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.

Decorative Image: 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 ).

Databases for Literature Searching
Database Scope

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!

View our example in PICO format

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

PICO Elements and Examples for sample research question
Element Example

atient(s) /  opulation(s)

 

patients 65 years and older

ntervention(s)

 

influenza vaccine

omparison(s)

 

not applicable

utcome(s)

 

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 on the HSL PubMed Ten Tips Legacy Guide .

Note: Controlled vocabulary may be outdated, and some databases allow users to submit requests to update terminology.

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.

Formatting controlled vocabulary in different databases
Database Controlled Vocabulary Indicated By Example
PubMed (MEDLINE)

Medical Subject Headings (MeSH)

[MeSH] "Influenza Vaccines"[Mesh]
Embase EMTREE /exp 'influenza vaccine'/exp
CINAHL CINAHL Headings MH or MM (MH "Influenza Vaccine")
PsycINFO APA Thesaurus DE DE "Influenza"
Sociological Abstracts Thesaurus of Sociological Indexing Terms MAINSUBJECT.EXACT MAINSUBJECT.EXACT("Influenza")

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.

Display Controlled Vocabulary and Keywords for our example PICO

Controlled vocabulary and keywords for sample research question
PICO Element Example Controlled Vocabulary Synonyms/Keyword Terms

 

 

 

atient(s) /  opulation(s)

 

 

 

 

patients 65 years and older

 

 

 

 

"Aged"[Mesh]

elder

elders

elderly

aged

aging

geriatric

geriatrics

gerontology

gerontological

senior citizen

senior citizens

older adult

older adults

older individuals

older patients

older people

older persons

advancing age

 

 

 

ntervention(s)

 

 

 

 

influenza vaccine

 

 

 

"Influenza Vaccines"[Mesh]

influenza vaccines

flu vaccine

flu vaccines

influenza virus vaccine

influenza virus vaccines

((flu OR influenza) AND (vaccine OR vaccines OR vaccination OR immunization))

omparison(s)

 

not applicable

 

-

 

-

utcome(s)

 

pneumonia

 

"Pneumonia"[Mesh]

pneumonias

pulmonary inflammation

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])))

View field tags for several health databases

Field tags in health sciences databases
Database Select Field Tags Example
PubMed (MEDLINE)
Embase
CINAHL, PsycInfo, & other EBSCO databases
Sociological Abstracts & other Proquest 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

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 .

Search strategy for sample research question
PICO Element Example Controlled Vocabulary (Database-Specific) Synonyms/Keyword Terms Sample Search Strategies (Combine Controlled Vocabulary & Keywords)

 

 

 

atient(s) /  opulation(s)

 

 

 

 

patients 65 years and older

 

 

 

 

"Aged"[Mesh]

elder

elders

elderly

aged

aging

geriatric

geriatrics

gerontology

gerontological

senior citizen

senior citizens

older adult

older adults

older patients

advancing age

 

 

 

(“Aged”[Mesh] OR elder[tiab] OR elders[tiab] OR elderly[tw] OR aged[tw] OR aging[tiab] OR “older adult”[tw] OR “older adults”[tw] OR “older patients”[tw] OR “advancing age”[tiab] OR geriatric[tw] OR geriatrics[tw] OR gerontology[tw] OR gerontological[tw] OR “senior citizen”[tw] OR “senior citizens”[tw])

 

 

 

ntervention(s)

 

 

 

 

influenza vaccine

 

 

 

"Influenza Vaccines"[Mesh]

influenza vaccines

flu vaccine

flu vaccines

influenza virus vaccine

influenza virus vaccines

(flu OR influenza) AND (vaccine OR vaccines OR vaccination OR immunization)

 

 

("Influenza Vaccines"[Mesh] 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])))

omparison(s)

 

not applicable

 

-

 

-

 

-

utcome(s)

 

pneumonia

 

"Pneumonia"[Mesh]

pneumonias

pulmonary inflammation

 

("Pneumonia"[Mesh] OR pneumonia[tw] OR pneumonias[tw] OR “pulmonary inflammation”[tw])

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.

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  • Last Updated: Jul 15, 2024 4:55 PM
  • URL: https://guides.lib.unc.edu/systematic-reviews

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Methodology

  • How to Write a Literature Review | Guide, Examples, & Templates

How to Write a Literature Review | Guide, Examples, & Templates

Published on January 2, 2023 by Shona McCombes . Revised on September 11, 2023.

What is a literature review? A literature review is a survey of scholarly sources on a specific topic. It provides an overview of current knowledge, allowing you to identify relevant theories, methods, and gaps in the existing research that you can later apply to your paper, thesis, or dissertation topic .

There are five key steps to writing a literature review:

  • Search for relevant literature
  • Evaluate sources
  • Identify themes, debates, and gaps
  • Outline the structure
  • Write your literature review

A good literature review doesn’t just summarize sources—it analyzes, synthesizes , and critically evaluates to give a clear picture of the state of knowledge on the subject.

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Table of contents

What is the purpose of a literature review, examples of literature reviews, step 1 – search for relevant literature, step 2 – evaluate and select sources, step 3 – identify themes, debates, and gaps, step 4 – outline your literature review’s structure, step 5 – write your literature review, free lecture slides, other interesting articles, frequently asked questions, introduction.

  • Quick Run-through
  • Step 1 & 2

When you write a thesis , dissertation , or research paper , you will likely have to conduct a literature review to situate your research within existing knowledge. The literature review gives you a chance to:

  • Demonstrate your familiarity with the topic and its scholarly context
  • Develop a theoretical framework and methodology for your research
  • Position your work in relation to other researchers and theorists
  • Show how your research addresses a gap or contributes to a debate
  • Evaluate the current state of research and demonstrate your knowledge of the scholarly debates around your topic.

Writing literature reviews is a particularly important skill if you want to apply for graduate school or pursue a career in research. We’ve written a step-by-step guide that you can follow below.

Literature review guide

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See an example

how to search for literature review articles

Writing literature reviews can be quite challenging! A good starting point could be to look at some examples, depending on what kind of literature review you’d like to write.

  • Example literature review #1: “Why Do People Migrate? A Review of the Theoretical Literature” ( Theoretical literature review about the development of economic migration theory from the 1950s to today.)
  • Example literature review #2: “Literature review as a research methodology: An overview and guidelines” ( Methodological literature review about interdisciplinary knowledge acquisition and production.)
  • Example literature review #3: “The Use of Technology in English Language Learning: A Literature Review” ( Thematic literature review about the effects of technology on language acquisition.)
  • Example literature review #4: “Learners’ Listening Comprehension Difficulties in English Language Learning: A Literature Review” ( Chronological literature review about how the concept of listening skills has changed over time.)

You can also check out our templates with literature review examples and sample outlines at the links below.

Download Word doc Download Google doc

Before you begin searching for literature, you need a clearly defined topic .

If you are writing the literature review section of a dissertation or research paper, you will search for literature related to your research problem and questions .

Make a list of keywords

Start by creating a list of keywords related to your research question. Include each of the key concepts or variables you’re interested in, and list any synonyms and related terms. You can add to this list as you discover new keywords in the process of your literature search.

  • Social media, Facebook, Instagram, Twitter, Snapchat, TikTok
  • Body image, self-perception, self-esteem, mental health
  • Generation Z, teenagers, adolescents, youth

Search for relevant sources

Use your keywords to begin searching for sources. Some useful databases to search for journals and articles include:

  • Your university’s library catalogue
  • Google Scholar
  • Project Muse (humanities and social sciences)
  • Medline (life sciences and biomedicine)
  • EconLit (economics)
  • Inspec (physics, engineering and computer science)

You can also use boolean operators to help narrow down your search.

Make sure to read the abstract to find out whether an article is relevant to your question. When you find a useful book or article, you can check the bibliography to find other relevant sources.

You likely won’t be able to read absolutely everything that has been written on your topic, so it will be necessary to evaluate which sources are most relevant to your research question.

For each publication, ask yourself:

  • What question or problem is the author addressing?
  • What are the key concepts and how are they defined?
  • What are the key theories, models, and methods?
  • Does the research use established frameworks or take an innovative approach?
  • What are the results and conclusions of the study?
  • How does the publication relate to other literature in the field? Does it confirm, add to, or challenge established knowledge?
  • What are the strengths and weaknesses of the research?

Make sure the sources you use are credible , and make sure you read any landmark studies and major theories in your field of research.

You can use our template to summarize and evaluate sources you’re thinking about using. Click on either button below to download.

Take notes and cite your sources

As you read, you should also begin the writing process. Take notes that you can later incorporate into the text of your literature review.

It is important to keep track of your sources with citations to avoid plagiarism . It can be helpful to make an annotated bibliography , where you compile full citation information and write a paragraph of summary and analysis for each source. This helps you remember what you read and saves time later in the process.

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To begin organizing your literature review’s argument and structure, be sure you understand the connections and relationships between the sources you’ve read. Based on your reading and notes, you can look for:

  • Trends and patterns (in theory, method or results): do certain approaches become more or less popular over time?
  • Themes: what questions or concepts recur across the literature?
  • Debates, conflicts and contradictions: where do sources disagree?
  • Pivotal publications: are there any influential theories or studies that changed the direction of the field?
  • Gaps: what is missing from the literature? Are there weaknesses that need to be addressed?

This step will help you work out the structure of your literature review and (if applicable) show how your own research will contribute to existing knowledge.

  • Most research has focused on young women.
  • There is an increasing interest in the visual aspects of social media.
  • But there is still a lack of robust research on highly visual platforms like Instagram and Snapchat—this is a gap that you could address in your own research.

There are various approaches to organizing the body of a literature review. Depending on the length of your literature review, you can combine several of these strategies (for example, your overall structure might be thematic, but each theme is discussed chronologically).

Chronological

The simplest approach is to trace the development of the topic over time. However, if you choose this strategy, be careful to avoid simply listing and summarizing sources in order.

Try to analyze patterns, turning points and key debates that have shaped the direction of the field. Give your interpretation of how and why certain developments occurred.

If you have found some recurring central themes, you can organize your literature review into subsections that address different aspects of the topic.

For example, if you are reviewing literature about inequalities in migrant health outcomes, key themes might include healthcare policy, language barriers, cultural attitudes, legal status, and economic access.

Methodological

If you draw your sources from different disciplines or fields that use a variety of research methods , you might want to compare the results and conclusions that emerge from different approaches. For example:

  • Look at what results have emerged in qualitative versus quantitative research
  • Discuss how the topic has been approached by empirical versus theoretical scholarship
  • Divide the literature into sociological, historical, and cultural sources

Theoretical

A literature review is often the foundation for a theoretical framework . You can use it to discuss various theories, models, and definitions of key concepts.

You might argue for the relevance of a specific theoretical approach, or combine various theoretical concepts to create a framework for your research.

Like any other academic text , your literature review should have an introduction , a main body, and a conclusion . What you include in each depends on the objective of your literature review.

The introduction should clearly establish the focus and purpose of the literature review.

Depending on the length of your literature review, you might want to divide the body into subsections. You can use a subheading for each theme, time period, or methodological approach.

As you write, you can follow these tips:

  • Summarize and synthesize: give an overview of the main points of each source and combine them into a coherent whole
  • Analyze and interpret: don’t just paraphrase other researchers — add your own interpretations where possible, discussing the significance of findings in relation to the literature as a whole
  • Critically evaluate: mention the strengths and weaknesses of your sources
  • Write in well-structured paragraphs: use transition words and topic sentences to draw connections, comparisons and contrasts

In the conclusion, you should summarize the key findings you have taken from the literature and emphasize their significance.

When you’ve finished writing and revising your literature review, don’t forget to proofread thoroughly before submitting. Not a language expert? Check out Scribbr’s professional proofreading services !

This article has been adapted into lecture slides that you can use to teach your students about writing a literature review.

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A literature review is a survey of scholarly sources (such as books, journal articles, and theses) related to a specific topic or research question .

It is often written as part of a thesis, dissertation , or research paper , in order to situate your work in relation to existing knowledge.

There are several reasons to conduct a literature review at the beginning of a research project:

  • To familiarize yourself with the current state of knowledge on your topic
  • To ensure that you’re not just repeating what others have already done
  • To identify gaps in knowledge and unresolved problems that your research can address
  • To develop your theoretical framework and methodology
  • To provide an overview of the key findings and debates on the topic

Writing the literature review shows your reader how your work relates to existing research and what new insights it will contribute.

The literature review usually comes near the beginning of your thesis or dissertation . After the introduction , it grounds your research in a scholarly field and leads directly to your theoretical framework or methodology .

A literature review is a survey of credible sources on a topic, often used in dissertations , theses, and research papers . Literature reviews give an overview of knowledge on a subject, helping you identify relevant theories and methods, as well as gaps in existing research. Literature reviews are set up similarly to other  academic texts , with an introduction , a main body, and a conclusion .

An  annotated bibliography is a list of  source references that has a short description (called an annotation ) for each of the sources. It is often assigned as part of the research process for a  paper .  

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How to undertake a literature search: a step-by-step guide

Affiliation.

  • 1 Literature Search Specialist, Library and Archive Service, Royal College of Nursing, London.
  • PMID: 32279549
  • DOI: 10.12968/bjon.2020.29.7.431

Undertaking a literature search can be a daunting prospect. Breaking the exercise down into smaller steps will make the process more manageable. This article suggests 10 steps that will help readers complete this task, from identifying key concepts to choosing databases for the search and saving the results and search strategy. It discusses each of the steps in a little more detail, with examples and suggestions on where to get help. This structured approach will help readers obtain a more focused set of results and, ultimately, save time and effort.

Keywords: Databases; Literature review; Literature search; Reference management software; Research questions; Search strategy.

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how to search for literature review articles

How To Find A-Grade Literature For Review

Sourcing, evaluating and organising.

By: David Phair (PhD) and Peter Quella (PhD) | January 2022

As we’ve discussed previously on our blog and YouTube channel, the first step of the literature review process is to source high-quality , relevant resources for your review, and to catalogue these pieces of literature in a systematic way so that you can digest and synthesise all the content efficiently.

In this article, we’ll look discuss 6 important things to keep in mind for the initial stage of your literature review so that you can source high-quality, relevant resources, quickly and efficiently. Let’s get started!

Overview: Literature Review Sourcing

  • Develop and follow a clear literature search strategy
  • Understand and use different types of literature correctly
  • Carefully evaluate the quality of your potential sources
  • Use a reference manager and a literature catalogue
  • Read as broadly and comprehensively as possible
  • Keep your golden thread front of mind throughout the process

1. Have a clear literature search strategy

As with any task in the research process, you need to have a clear plan of action before you get started, or you’ll end up wasting a lot of time and energy. So, before you begin your literature review , it’s useful to develop a simple search strategy . Broadly speaking, a good literature search strategy should include the following steps:

Step one – Clearly identify your golden thread

Your golden thread consists of your research aims , research objectives and research questions . These three components should be tightly aligned to form the focus of your research. If you’re unclear what your research aims and research questions are, you’re not going to have a clear direction when trying to source literature. As a result, you’re going to waste a lot of time reviewing irrelevant resources.

So, make sure that you have clarity regarding your golden thread before you start searching for literature. Of course, your research aims, objectives and questions may evolve or shift as a result of the literature review process (in fact, this is quite common), but you still need to have a clear focus to get things started.

Step two – Develop a keyword/keyphrase list

Once you’ve clearly articulated your golden thread in terms of the research aims, objectives and questions, the next step is to develop a list of keywords or keyphrases, based on these three elements (the golden thread). You’ll also want to include synonyms and alternative spellings (for example, American vs British English) in your list.

For example, if your research aims and research questions involve investigating organisational trust , your keyword list might include:

  • Organisational trust
  • Organizational trust (US spelling)
  • Consumer trust
  • Brand trust
  • Online trust

When it comes to brainstorming keywords, the more the better . Don’t hold back at this stage. You’ll quickly find out which ones are useful, and which aren’t when you start searching. So, it’s best to just go as broad as possible here to ensure you cast a wide net.

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Step three – Identify the relevant databases

Now that you’ve got a comprehensive set of keywords, the next step is to identify which literature databases will be most useful and relevant for your particular study. There are hundreds, if not thousands of databases out there, and they are often subject or discipline-specific . For example, within the medicine space, Medline is a popular one.

To identify relevant databases, it’s best to speak to your research advisor/supervisor, Grad Coach or a librarian at your university library. Oftentimes, a quick chat with a skilled librarian can yield tremendous insight. Don’t be shy to ask – chances are, they’ll be thrilled that you asked!

At this stage, you might be asking, “why not just use Google Scholar?”. Of course, an academic search engine like Google Scholar will be useful in terms of getting started and finding a broad range of resources, but it won’t always present every possible resource  or the best quality resources. It also has limited filtering options compared to some of the specialist databases, so you shouldn’t rely purely on Google Scholar.

Step four – Use Boolean operators to refine your search

Once you’ve identified your keywords and databases, it’s time to start searching for literature – hooray! However, you’ll quickly find that there is a seemingly endless number of journal articles to sift through, and you have limited time to work through the literature. So, you’ll need to get smart about how you use these databases – enter Boolean operators.

Boolean operators are special characters that allow you to refine your search. Common operators include:

  • AND – only show results that contain both X and Y
  • OR – show results that contain X or Y
  • NOT – show results that include X, but not Y

These operators are incredibly useful, especially when there are topics that are very similar to yours but are not relevant . For example, if you’re researching something about the growth of apples, you’ll want to exclude all literature related to Apple, the company. Boolean operators allow you to cut out the irrelevant content and improve the signal to noise ratio in your search.

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2. Use different types of literature correctly

Once you start searching for literature, you’ll quickly notice that there are different “types” of resources that come up. It’s important to understand the different types of literature available to you and how to use each of them appropriately.

Generally speaking, you’ll find three categories of literature:

Primary literature

Secondary literature

Tertiary literature

Primary literature refers to journal articles , typically peer reviewed, which document a study that was undertaken, where data were collected and analysed, and findings were discussed. For example, a journal article that involves the collection and analysis of survey data to identify differences in personality between two groups of people.

Primary literature should, ideally, form the foundation of your literature review – the bread and butter, so to speak. You’ll likely refer to many of the arguments made and findings identified in these types of articles to build your own arguments throughout your literature review. You’ll also rely on these types of articles for theoretical models and frameworks, which may form the foundation of your own proposed framework, depending on the nature of your research.

Lastly, primary literature can be a useful source of measurement scales for quantitative studies. For example, many journal articles will include a copy of the survey measures they used at the end of the article, which will typically be reliable and valid. You can either use these “as is” or as a foundation for your own survey measures .

So, long story short, you’ll need a good stockpile of these types of resources. They are, admittedly, more “dense” and challenging to digest than the other types of literature, but taking the time to work through them will pay off greatly.

Secondary literature refers to journal articles that summarise and integrate the findings from primary literature. For example, you’ll likely find “review of the literature” type journal articles which provide an overview of the current state of the research (at the time of publication, of course).

Secondary literature is very useful for orienting yourself with regards to the current state of knowledge and identifying key researchers , seminal works and so on. In other words, they’re a good tool to make sure you’ve got a broad, comprehensive view of what all is out there. They’re not going to give you the level of detail that primary literature will (and they’ll likely be a bit outdated), but they’ll point you in the right direction.

In practical terms, it’s a good idea to start by reviewing secondary literature-type articles to help you get a bird’s eye view of the landscape and then dive deeper into the primary literature to get a grasp of the specifics and to bring your knowledge up to date with the most current studies.

The final category of literature refers to sources that would be considered less academic and scientifically rigorous in nature, but up to date and highly relevant. For example, sources such as current industry and country reports published by management consulting groups, news articles, blog posts and so on.

While these sources are not as credible and trustworthy as journal articles (especially peer-reviewed ones), they can provide very up to date information , whereas academic research tends to roll out quite slowly. Therefore, they can be very useful for contextualising your research topic and/or demonstrating a current trend. Quite often, you’d cite these types of sources in your introduction chapter rather than your literature review chapter, but you may still have use for them in the latter.

In summary, it’s important to understand the three different types of literature – primary, secondary and tertiary, and use them appropriately in your dissertation, thesis or research project.

It’s important to understand the different types of literature available to you and how to use each of them appropriately in your literature review.

3. Carefully evaluate the quality of your sources

As we’ve alluded to, not all literature is created equally. Not only does literature vary in terms of type (i.e., primary vs secondary), it also varies in terms of overall quality .

Simply put, all sources exist on a quality spectrum . On the high end of the spectrum are peer-reviewed articles published in popular, credible journals. Next are journal articles that are not peer-reviewed, or that are published in lower quality or lesser-known journals. In the middle are sources like textbooks and reports by professional organisations (e.g., management consulting firms). On the low end are sources like newspapers, blog posts and social media posts.

As you can probably see, this loosely reflects the categories we mentioned previously (primary, secondary and tertiary literature), so there is once again a trade-off between quality and recency . Therefore, you need to carefully evaluate the quality of each potential source and let this inform how you use it in your literature review. Importantly, this doesn’t mean that you can’t include a newspaper article or blog post as a source – it just means that you shouldn’t rely too heavily on these types of courses as the core of your argument.

When evaluating journal articles, you can consider their citation count (i.e., the number of other articles that reference them) as a quality indicator. But keep in mind that citation count is a product of many factors , including the popularity of the article, the popularity of the research field and most importantly, time. In other words, it’s natural for newer articles to have lower citation counts. This is useful to keep in mind, as you ideally want to focus on more recent literature (published within the last 3-5 years) in your literature review.

In summary, aim to focus on higher-quality literature , especially when you’re building core arguments in your literature review. You don’t, for example, want to make an argument regarding the importance and novelty of your research (i.e., its justification) based on some blogger’s opinion.

All literature and resources exist on a quality spectrum, ranging from high-quality (typically less recent) to low-quality (oftentimes recent).

4. Use a reference manager and literature catalogue

As you review the literature and build your collection of potential sources, you’ll need a way to stay on top of all the details. To this end, it’s essential that you make use of both a reference manager and a literature catalogue . Let’s take a look at each of these.

The reference manager

Reference management software helps you store the reference information for each of your articles and manages the citation and reference list building task as you write up your actual literature review chapter. In other words, a reference manager ensures that your citations and reference list are correctly formatted in the reference style required by your university – e.g., Harvard, APA , MLA, etc.

Using a reference manager saves you the hassle of trying to manually type out your in-text citations and reference list, which you’re bound to mess up in some way. A simple comma out of place, incorrect italicisation or boldfacing can result in you losing marks, and that’s highly likely when you’re dealing with a large number of references. So, it just makes sense to use a piece of software for this task.

The good news is that there are loads of options , many of which are free . For new researchers, we usually recommend Mendeley or Zotero . So, don’t waste your time trying to manage your references manually – get yourself a reference manager ASAP.

The literature catalogue

The second tool you’ll need is a literature catalogue. This is simply an Excel document that you can easily compile yourself (or download our free one here ), where you list and categorise all your literature. You might doubt whether it’s really necessary to have a separate catalogue when you’ve already logged your reference data in a reference manager, but trust us, you’re going to need it. It’s quite common that throughout the literature review process, you’ll review hundreds of articles , so it’s simply impossible that you’ll remember all the details.

What makes a literature catalogue extremely powerful is that you can store as much information as you want for each piece of literature that you include (whereas a reference manager only includes basic fields). Typically, you would include things like:

  • Title of the article
  • One-line summary of the research
  • Key findings and takeaways
  • Context (i.e. where did it take place)
  • Useful quotes
  • Methodology (e.g. qualitative, quantitative or mixed methods)
  • Category (you can customise as many categories as makes sense for you)
  • Quality of resource
  • Type of literature (e.g. primary, secondary or tertiary)

These are just some examples – ultimately you need to customise your catalogue to suit your needs. But, as you can see, the more detailed you get, the more useful your catalogue will become when it’s time to synthesise the research and write up your literature. For example, you could quickly filter the catalogue to display all papers that support a certain hypothesis, that argue in a specific direction, or that were written at a certain time.

5. Read widely (and efficiently)

As we’ve discussed in other posts , the purpose of the literature review chapter is to present and synthesise the current state of knowledge in relation to your research aims, objectives and research questions. To do this, you’ll need to read as broadly and comprehensively as possible. You’ll need to demonstrate to your marker that you “know your stuff” and have a strong understanding of the relevant literature.

Ideally, your literature review should include an eclectic mix of research that features multiple perspectives . In other words, you need to avoid getting tunnel vision and running down one narrow stream of literature. Ideally, you want to highlight both the agreements and disagreements in the literature to show that you’ve got a well-balanced view of the situation.

If your topic is particularly novel and there isn’t a lot of literature available, you can focus your efforts on adjacent literature . For example, if you’re researching factors that cultivate organisational trust in Germany, but there’s very little literature on this, you can draw on US and UK-based studies to form your theoretical foundation. Similarly, if you’re investigating an occurrence in an under-researched industry, you can look at other industries for literature.

As you read each journal article, be sure to scan the reference list for further reading (this technique is called “snowballing”). By doing this, you will quickly identify key literature within a topic area and fast-track your literature review process. You can also check which articles have cited any given article using Google Scholar, which will give you a “forward view” in terms of the progress of the literature.

Given that you’ll need to work through a large amount of literature, it’s useful to adopt a “strategic skimming ” approach when you’re initially assessing articles, so that you don’t need to read the entire journal article . In practical terms, this means you can focus on just the title and abstract at first, and if the article seems relevant based on those, you can jump to the findings section and limitations section . These sections will give you a solid indicator as to whether the resource is relevant to your study, which you can then shortlist for full reading.

eclectic mix of research that features multiple perspectives to avoid tunnel vision.

6. Keep your golden thread front of mind

Your golden thread (i.e., your research aims, objectives and research questions) needs to guide every decision you make throughout your dissertation, thesis, or research project. This is especially true in the literature review stage, as the golden thread should act as a litmus test for relevance whenever you’re reviewing potential articles or resources. In other words, if an article doesn’t relate to your golden thread, its probably not worth spending time on.

Keep in mind that your research aims, objectives and research questions may evolve as a result of the literature review process. For example, you may find that after reviewing the literature in more depth, your topic focus is not as novel as you originally thought, or that there’s an adjacent area that is more deserving of investigation. This is perfectly natural, so don’t be surprised if your focus shifts somewhat during the review process. Just remember to update your literature review in this case and be sure to update any previous chapters so that your document has a consistent focus throughout.

Wrapping up

In this article, we covered 6 pointers to help you find and evaluate high-quality resources for your literature review. To recap:

  • Understand and use different types of literature for the right purpose

If you have any questions, please feel free to leave a comment . Alternatively, if you’d like hands-on help with your literature review, be sure to check out our 1-on-1 private coaching services here.

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  • UConn Library
  • Literature Review: The What, Why and How-to Guide
  • Strategies to Find Sources

Literature Review: The What, Why and How-to Guide — Strategies to Find Sources

  • Getting Started
  • Introduction
  • How to Pick a Topic
  • Evaluating Sources & Lit. Reviews
  • Tips for Writing Literature Reviews
  • Writing Literature Review: Useful Sites
  • Citation Resources
  • Other Academic Writings

The Research Process

Interative Litearture Review Research Process image (Planning, Searching, Organizing, Analyzing and Writing [repeat at necessary]

Planning : Before searching for articles or books, brainstorm to develop keywords that better describe your research question.

Searching : While searching, take note of what other keywords are used to describe your topic, and use them to conduct additional searches

     ♠ Most articles include a keyword section

     ♠ Key concepts may change names throughout time so make sure to check for variations

Organizing : Start organizing your results by categories/key concepts or any organizing principle that make sense for you . This will help you later when you are ready to analyze your findings

Analyzing : While reading, start making notes of key concepts and commonalities and disagreement among the research articles you find.

♠ Create a spreadsheet  to record what articles you are finding useful and why.

♠ Create fields to write summaries of articles or quotes for future citing and paraphrasing .

Writing : Synthesize your findings. Use your own voice to explain to your readers what you learned about the literature on your topic. What are its weaknesses and strengths? What is missing or ignored?

Repeat : At any given time of the process, you can go back to a previous step as necessary.

Advanced Searching

All databases have Help pages that explain the best way to search their product. When doing literature reviews, you will want to take advantage of these features since they can facilitate not only finding the articles that you really need but also controlling the number of results and how relevant they are for your search. The most common features available in the advanced search option of databases and library online catalogs are:

  • Boolean Searching (AND, OR, NOT): Allows you to connect search terms in a way that can either limit or expand your search results 
  • Proximity Searching (N/# or W/#): Allows you to search for two or more words that occur within a specified number of words (or fewer) of each other in the database
  • Limiters/Filters : These are options that let you control what type of document you want to search: article type, date, language, publication, etc.
  • Question mark (?) or a pound sign (#) for wildcard: Used for retrieving alternate spellings of a word: colo?r will retrieve both the American spelling "color" as well as the British spelling "colour." 
  • Asterisk (*) for truncation: Used for retrieving multiple forms of a word: comput* retrieves computer, computers, computing, etc.

Want to keep track of updates to your searches? Create an account in the database to receive an alert when a new article is published that meets your search parameters!

  • EBSCOhost Advanced Search Tutorial Tips for searching a platform that hosts many library databases
  • Library's General Search Tips Check the Search tips to better used our library catalog and articles search system
  • ProQuest Database Search Tips Tips for searching another platform that hosts library databases

There is no magic number regarding how many sources you are going to need for your literature review; it all depends on the topic and what type of the literature review you are doing:

► Are you working on an emerging topic? You are not likely to find many sources, which is good because you are trying to prove that this is a topic that needs more research. But, it is not enough to say that you found few or no articles on your topic in your field. You need to look broadly to other disciplines (also known as triangulation ) to see if your research topic has been studied from other perspectives as a way to validate the uniqueness of your research question.

► Are you working on something that has been studied extensively? Then you are going to find many sources and you will want to limit how far back you want to look. Use limiters to eliminate research that may be dated and opt to search for resources published within the last 5-10 years.

  • << Previous: How to Pick a Topic
  • Next: Evaluating Sources & Lit. Reviews >>
  • Last Updated: Sep 21, 2022 2:16 PM
  • URL: https://guides.lib.uconn.edu/literaturereview

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  • v.106(4); 2018 Oct

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

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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.

An external file that holds a picture, illustration, etc.
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

PubMedOvidEBSCOhostEmbase.comProQuest
Title/abstract[tiab] ().ab,ti.TI () OR AB () ():ab,tiAB,TI()
All fields[All Fields].af. ALL
Thesaurus term[mesh:noexp]…/MH “…”‘…’/deMESH(…)
Including narrower[mesh]exp …/MH “…+”‘…’/expMESH#(…)
Combined subheading [mesh]exp …/ MH “…+/ ”‘…’/exp/dm_ MESH(… LNK ..)
Free subheading[sh] .xs. or .fs. MW:lnk
Publication type[pt] .pt. or exp / PT:it RTYPE
Proximity ADJnNnNEAR/n-NEXT/nN/n
Exact phrase“double quotes”No quotes needed“double quotes”‘single quotes’“double quotes”
Truncated phraseUse-hyphen*No quote*No quote*‘single quote*’“Double quote*”
TruncationEndEnd/ midEnd/ midEnd/ midEnd / mid / start
Infinite** or $***
0 or 1 character?#$1
1 character#?? ?
Added to database sinceyyyy/mm/dd:yyyy/mm/dd [edat] (or [mhda])limit #N to rd=yyyymmdd-yyyymmdd EM yyyymmdd-yyyymmdd[dd-mm-yyyy]/sdLUPD(yyyymmdd)
Publication period (years)yyyy:yyyy[dp]limit #N to yr=yyyy-yyyy PY yyyy-yyyy[yyyy-yyyy]/pyYR (yyyy-yyyy)
Record sets#11 S1#1S1

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.

An external file that holds a picture, illustration, etc.
Object name is jmla-106-531-f002.jpg

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.

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  • Correction 09 December 2020

How to write a superb literature review

Andy Tay is a freelance writer based in Singapore.

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Literature reviews are important resources for scientists. They provide historical context for a field while offering opinions on its future trajectory. Creating them can provide inspiration for one’s own research, as well as some practice in writing. But few scientists are trained in how to write a review — or in what constitutes an excellent one. Even picking the appropriate software to use can be an involved decision (see ‘Tools and techniques’). So Nature asked editors and working scientists with well-cited reviews for their tips.

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doi: https://doi.org/10.1038/d41586-020-03422-x

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Correction 09 December 2020 : An earlier version of the tables in this article included some incorrect details about the programs Zotero, Endnote and Manubot. These have now been corrected.

Hsing, I.-M., Xu, Y. & Zhao, W. Electroanalysis 19 , 755–768 (2007).

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Ledesma, H. A. et al. Nature Nanotechnol. 14 , 645–657 (2019).

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Choi, Y. & Lee, S. Y. Nature Rev. Chem . https://doi.org/10.1038/s41570-020-00221-w (2020).

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