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Systematic reviews, scoping reviews and other evidence syntheses

Overview of evidence synthesis reviews and relevant strategies, tools and resources.

Develop a search strategy

Search strategy

  • refers to the specialized combination of keywords, subject headings, search syntax and logic operators used to retrieve the optimal number of studies for an evidence synthesis project
  • your search strategy should be comprehensive and designed to collect all available relevant evidence on a research question, including peer-reviewed and grey literature; ideally, unpublished and non-English sources should be included to reduce bias
  • multiple sources must be searched because:
    • content varies across different subject databases - no one database includes all relevant content
    • Not all research is published in journals, or published at all

Reporting search strategies

Search strategies are critical components of evidence synthesis research and are:

  • documented and described in the Methods section of published reviews
  • included as an Appendix in published reviews

Follow guidance from PRISMA on how to report search strategies in a systematic review:

Rethlefsen, M.L., Kirtley, S., Waffenschmidt, S. et al. PRISMA-S: an extension to the PRISMA Statement for Reporting Literature Searches in Systematic Reviews. Syst Rev 10, 39 (2021). https://doi.org/10.1186/s13643-020-01542-z

Expert guidance from librarians

Key evidence synthesis organizations (IOM, Cochrane, CIHR, Campbell Collaboration) highlight the importance of librarian expertise in developing search strategies which are:

  • Comprehensive
  • Free from bias as possible
  • Transparent 
  • Reproducible

Building a search strategy

Expect an iterative process, beginning with preliminary searching before developing a final comprehensive search strategy.

  • Preliminary searching:
    • Identify key terms, synonyms and subject headings to use in your search
    • Use your target articles to help identify a primary subject database to begin building your search
  • Comprehensive searching:
    • Develop a full search strategy including:
      • keywords, synonyms and subject headings
      • appropriate search syntax and logic operators
    • Identify appropriate subject databases for your topic (e.g. MEDLINE, CINAHL)
      • important to search in multiple databases because each database has unique content; evidence syntheses searching must be as comprehensive as possible
    • Run a trial search in one database to test its efficacy; revise search as needed
    • Run searches in other relevant databases

Supplementary searching

  • Use modified search strategies to find crucial sources of information including:
    • Preprint articles
    • Grey literature
    • Theses and dissertations
    • Clinical trials
  • Hand search additional relevant non-indexed journals or conference proceedings, if appropriate
  • Contact experts, including corresponding authors of included studies, to find out about additional or ongoing studies

Citation Tracing

  • Using the reference lists of your final included articles, conduct forward and backward citation tracing to capture additional articles

Document your searches

  • Best practices for evidence synthesis research require transparency and reproducibility: documentation supports this
  • Searching is iterative and requires a significant investment of time: you will need to refer back to your search decisions months later and may need to respond to reviewers' questions about those decisions
  • Reporting standards require you to report on search strategies in the methods section of your review project

  • Use this spreadsheet:

Search techniques

Building a search strategy

You will develop a sophisticated combination of keywords, subject headings, search syntax and logic operators to retrieve the optimal number of studies for an evidence synthesis project

  • Keywords -- natural language words describing the main concepts in your research e.g. if your research question is investigating the effects of social media on adolescents' well-being, your topic keywords are:
    • social media
    • well-being
    • adolescents 
      • Always generate synonyms and related terms for each keyword to ensure you capture studies which investigate the same topic but use different descriptors e.g. adolescent - teenager, young adult, youth, etc.
  • Subject headings -- controlled vocabulary assigned by databases to articles about specific topics; helps broaden your search by ensuring you retrieve relevant studies which may use varying keywords

Search both keywords and subject headings

A search strategy must include both keywords and subject headings to be comprehensive and avoid these pitfalls:

  • natural language requires using multiple synonyms and variations
  • articles may not be correctly indexed
  • there may be no subject heading for your concept

Search syntax and techniques

  • Truncation -- use an asterisk (or other symbol specific to a database) to retrieve alternate word endings; broadens your search. e.g. teen* retrieves teens, teenager, teenagers, teenaged
  • Exact phrase -- enclose 2-3 words or a short phrase in quotes to ensure they are searched together instead of independently e.g. "social media"
  • Logic operators -- connect your search terms to both broaden and focus your search results
    • OR -- broadens search by retrieving ANY of the search terms e.g. teen* OR adolescent OR "young adult"
    • AND -- focuses search by retrieving ALL of the search terms e.g. teen* AND "social media"
    • NOT -- focuses search by removing irrelevant results e.g. "social media" NOT Twitter; use with care to avoid missing studies that include both of your terms
  • Adjacency searching -- use a proximity operator e.g. NEAR, WITHIN to find words which appear within a certain number of words of each other e.g. social N3 media; social W2 media
  • Parentheses -- connect related words e.g. ("social media" OR Twitter OR Facebook) AND (adolescent* OR "young adult*")

Search tutorials and tips

Documentation and tutorials for core and common subject resources

Systematic review database searching cheat sheet - health focus

Subject and niche databases

Translating searches to multiple databases

Note that evidence synthesis reviews require searching in multiple subject databases, each of which have their own: 

  • Subject focus  
  • Controlled vocabulary (subject headings)  
  • Syntax operators e.g. truncation, adjacency operators   

Once your foundational search strategy is complete, you will need to translate it using the language and syntax of other databases to ensure it is replicated as closely as possible. 

 

 

Limits and filters

Search limits

  • Database search results can often be limited by e.g. year, gender, species, etc.
  • Generally, avoid using these filters as they introduce bias into your search results
  • Database search limiters are appropriate when:
    • updating an existing search
    • updating an existing systematic review

Search filters

  • Also called hedges, these are pre-designed and validated search strings which allow you to filter your search results e.g. by study design without sacrificing sensitivity/specificity or introducing major sources of bias
  • Always use a critically appraised search filter from one of these organizations: