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Systematic Reviews: Grey Literature Searching

This guide provides information and resources which may be helpful when undertaking a systematic review or other type of knowledge synthesis.

Searching for Grey Literature

Cochrane indicates that a minimum of 3 databases should be searched (Section 4.3.1.1):

  1. CENTRAL
  2. MEDLINE
  3. Embase

Cochrane further recommends searching for grey literature categories including:

  • Ongoing studies and unpublished data sources (Section 4.3.2)
  • Trial registers and trials results registers (Section 4.3.3)
  • Regulatory agency sources and clinical study reports (Section 4.3.4)
  • Reports, dissertations, theses, databases of conference abstracts (Section 4.3.5)

Lefebvre C, Glanville J, Briscoe S, et al. Searching for and selecting studies. In: Higgins J, Thomas J, Chandler J, et al., eds. Cochrane Handbook for Systematic Reviews of Interventions. version 6.2: Cochrane; 2021.

Conference Abstracts: Include or Exclude?

The Mayo Clinic Libraries' systematic review search Request Form asks the requestor whether the librarian should include conference abstracts in the search results. "Cochrane and the United States National Academy of Sciences (NAS) both recommend always searching for and including conference abstracts in systematic reviews."1 Including abstracts in the search results helps mitigate publication bias by ensuring that data otherwise unpublished are captured in the review of the literature.

There are arguments against including conference abstracts in a systematic or knowledge synthesis review. Due to their brief nature, abstracts frequently lack adequate information regarding the study's design, methods, risk of bias, outcomes, or results; the review team must search for the full-length published paper (usually published following the conference) or contact the authors to obtain the information required for critical appraisal of the study. Additionally, the quality of the research presented in conference abstracts may be called into question. Many abstracts report preliminary/incomplete study results and are not peer-reviewed.

Scherer and Saldanha1 provide a flow chart for decision making regarding whether to include conference abstracts in a systematic review. Review teams must recognize how the inclusion or exclusion of conference abstracts affects the outcome of their review. Including conference abstracts may require extra effort on the part of the team, but excluding them results in the potential for bias.

Conference Abstracts Flow Chart

Recommended Sources for Locating Grey Literature

Cochrane Central Register of Controlled Trials is accessible to Mayo employees through the Ovid search interface available on the Mayo Libraries homepage.

For additional information on CENTRAL, see the Cochrane Handbook for Systematic Review of Interventions, 4.1.1.3 The Cochrane Central Register of Controlled Trials (CENTRAL).

Lefebvre C, Glanville J, Briscoe S, et al. Searching for and selecting studies. In: Higgins J, Thomas J, Chandler J, et al., eds. Cochrane Handbook for Systematic Reviews of Interventions. version 6.2: Cochrane; 2021.

References & Recommended Reading

1.  Scherer RW, Saldanha IJ. How should systematic reviewers handle conference abstracts? A view from the trenches. Systematic reviews. 2019;8(1):264.