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Search strategies

Searching systematically

Searching is a critical part of the systematic review - your search should try to identify all studies that meet the criteria of your review question. Errors in the search process could result in a biased or otherwise incomplete evidence base.

This series of tutorials from Cushing/Whitney Medical Library at Yale University provides a comprehensive overview of whats involved in a systematic search.

The first video introduces the concept of systematic review and makes a rough comparison between a search done for a systematic review and an ordinary literature search.

Used with permission from Cushing/Whitney Medical Library, Yale University

Structure your search

You will need to break your review question down into search concepts - the PICO model provides a useful structure:

P - What are the characteristics of the population you are looking at?
I - What main intervention/issue are you considering?
C - What will the intervention/issue be compared to?
O - What outcome does the intervention/issue hope to accomplish, measure, improve or affect?

You might also add:

Research design - which study type will give the highest level of evidence for the question?

You may not need to include all of the PICO concepts in your strategy.

Identify search limits/exclusion criteria

Think about the limits you intend to apply to your search

Criteria Questions to ask Advice from the Cochrane Handbook for Systemic Reviews of Interventions (2011)
Time Period Will your review be restricted by year of publication, or is it important that you cover all years? "Date restrictions should be applied only if it is known that relevant studies could only have been reported during a specific time period, for example if the intervention was only available after a certain time point."
Language Should you restrict to English language publications only? "Whenever possible review authors should attempt to identify and assess for eligibility all possibly relevant reports of trials irrespective of publication. No language restrictions should be included in the search strategy."
Publication type Are you restricting your search by publication type? "Format restrictions such as excluding letters are not recommended because letters may contain important additional information relating to an earlier trial report or new information about a trial not reported elserwhere.
Geographic considerations Are there any geographic considerations to include in your search strategy? For example, if you were researching Chinese herbal medicine, you would need to consult Chinese literature.

Search filters

These search strategies have been designed and tested by search experts - they include 'filters' to limit your search to specific research designs.

Document your search

Your search should be transparent and reproducible. The search process is iterative - you may discover new terms as you work through the review, and need to update your searches. It is important to keep a detailed record of your search, including:

  • Where you searched
  • When you carried out the search
  • The search strategy you used
  • The number of records you retrieved
  • How many were duplicate records

Testing your search

  • A good way to check if your search strategy is effective is to test it with a set of known relevant,  'gold standard', articles you would want your search to retrieve.
  • If your search misses relevant studies, examine these records, have you missed any subject headings or keywords in your strategy.
  • Remember there may be another reasons the article isn't being retrieved, not all journals are indexed in all databases so you may need to check this.
  • You may like to get your search strategy peer reviewed or looked over by a librarian, or by someone experienced in performing searches in your field. The PRESS guidelines can help with this: