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Systematic Review Search Strategies

Systematic review search strategies are often more complex than search strategies you may have used in the past. They can run to hundreds of lines long, depending on the research topic, as you will need to take into account all the synonyms and variant spellings for your topic.

You are likely to need various searching techniques, such as using truncation, wildcards, proximity and Boolean operators. You will also need to identify and be aware of what database fields you are searching e.g. title, abstract, author keywords and subject headings.

Your search strategy needs to be systematic, transparent and reproducible. This is becuase you will need to reproduce and translate it across different databases, and other researchers might wish to reproduce it in future. Your full search strategy should be published as part of your review, usually in the appendix.

Keyword searching and subject headings

Many literature databases used in systematic reviews have more than one way they can be searched. The most common default search option is keyword (free-text) searching, but you may also want to incorporate subject headings (e.g. MeSH in Medline) as well.

Keyword (free-text) searching

Searches for the words you input in the fields specified e.g. searches for the phrase “diabetes type 2” in the title and abstract. This will not find other variants such as diabetes mellitus type or diabetes type II etc. You would need to make sure your keyword strategy incorporates all the variations of the term/phrase you are interested in.

Subject headings

Also known as controlled vocabulary, subject terms, index headings, index terms. The most well-known are MeSH (Medical Subject Headings used in Medline, PubMed and the Cochrane Library) and Emtree, which isused in Embase. Other databases, such as APA PsycInfo and CINAHL, also use subject headings.

Searching for a subject heading finds any items in the database that have that subject heading attached to them. Not every keyword/phrase has an equivalent subject heading. Most systematic review search strategies use a mixture of keywords and subject headings to ensure you are finding all relevant results. You might find it helpful to look at some published systematic review search strategies related to your topic when formulating your search strategy..

How to plan a search strategy

Some guides we recommend to help you plan a search strategy:

Additional searching

As well as searching the main literature databases you may choose to search other resources, depending on your inclusion/exclusion criteria. Ensure you report all your searches and sources according to the PRISMA guidelines. 

Clinical Trial Registers

strategy when searching clinical trial registers, as they do not have the same search options as literature databases. Some suggested resources: 

Google Scholar

Google Scholar is a search engine rather than a literature database, but it is often used to check for items not found in the traditional database searches. It does not support systematic, reproducible searches, but can be used for additional searching. There are multiple research papers evaluating the use of Google Scholar in systematic reviews. For example:

  • Haddaway et al. 'The Role of Google Scholar in Evidence Reviews and Its Applicability to Grey Literature Searching', PLoS One, 10, 9,(2015), https://doi.org/10.1371/journal.pone.0138237.
  • Gusenbauer and Haddaway, 'Which academic search systems are suitable for systematic reviews or meta-analyses? Evaluating retrieval qualities of Google Scholar, PubMed, and 26 other resources', Res Synth Methods, 11, 2, (2020), pp. 181-217, https://doi.org/10.1002/jrsm.1378.
  • Briscoe et al. 'Feasibility and desirability of screening search results from Google Search exhaustively for systematic reviews: A cross-case analysis', Res Syn Meth, 14, 3, (2023), pp. 427-437, https://doi.org/10.1002/jrsm.1622. 

We recommend looking at the available methodological literature to inform appropriate use of Google Scholar in your systematic review. You will need to run a very simplified version of your search strategy when using Google Scholar.

Grey Literature

Grey literature is literature that is produced outside of traditional academic publishing routes. For example: theses, dissertations, reports issued by government/charity/academic/NHS and guidelines. Whether you choose to include grey literature in your review will depend on your area of research and inclusion/exclusion criteria. Some suggestions on where to look:

You will often need to run a very simplified version of your search when trying to find grey literature. 

Handsearching and checking reference lists

Other methods to identify additional studies for inclusion in systematic reviews are: 

  • Checking the reference list (bibliography) of any studies that are being included in the final review. 
  • Handsearching: the process of checking journal table of contents or conference proceedings to identify studies not found via database searching.
  • Appropriate journals/conferences may already be known and stated in the protocol before starting a review, or they may be identified during the screening process.
  • Cochrane Handsearching Guidelines.

Citation searching (snowballing)

This is more likely to be used in scoping reviews, which are looking at new, niche areas of research. A citation search allows you to check who has cited/references a key article since it has been published.

The major citation searching tools are:

  • Scopus 
  • Web of Science
  • Google Scholar. 

There are also AI Literature Mapping Tools [login required] that can assist with citation searching. We recommended evaluating the tools before use and, if you use these tools, you must report that in your review.

Training

The Library runs training for PGRs and staff on Search Strategies for Systematic Reviews in Life Sciences. Please contact us via librarians@leicester.ac.uk if you have any questions about training.

You can watch a recording of the Search Strategies for Systematic Reviews in Life Sciences training [November 2024 - login required]. University staff who are also NHS staff can also access services and resources provided by the UHL Libraries and Information Service.

Next: Systematic Review Screening Processes

Screening Processes
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