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Evidence Synthesis: Finding, appraising and synthesizing research

This guide provides an overview of evidence syntheses methods and points to resources to learn more about conducting the different steps of an evidence synthesis project.

The gray literature: What is it and how do I search it?

What is gray literature?  Gray literature is defined by the Cochrane Handbook for Systematic Reviews of Interventions as "...literature that is not formally published in sources such as books or journal articles." This can include information such as government reports, conference proceedings, graduate dissertations, unpublished clinical trials, and much more.  

Why search the gray literature if it hasn't been peer-reviewed?  There is a strong bias in scientific publishing toward publishing studies that show some sort of significant effect.  Meanwhile, many studies and trials that show no effect end up going unpublished.  But knowing that an intervention had no effect is just as important as knowing that it did have an effect when it comes to making decisions for practice and policy-making.  Thus, the grey literature can be critical.

How do I search the gray literature if it's unpublished and not in the scholarly databases?  Its true--finding gray literature and searching it systematically is challenging.  But there are a few approaches that you can take to add some structure to your search of this type of information:

  • Identify and record the sources you will search, including some indication of search terms used if appropriate.  The sources you search will be informed by your research question and where you expect to find information related to your question.
  • Try searching databases that specialize in gray literature like OpenMD, MedNar and BASE. OpenGrey, a previously available resource for gray literature, is also searchable from a locally maintained SQL database.  For more resources, see the box below.
  • Conference proceedings:  Identify professional organizations that have conferences at which researchers might be presenting work related to your topic.  Search those conference proceedings on the organization's website or by contacting organizational boards for access to past proceedings that may not be online.
  • Theses and dissertations:  There are a number of databases dedicated to theses and dissertations, which you can search using your search terms.  See the box below for links to these resources. 
  • Identify government agencies, and international and non-governmental organizations, that might publish technical papers and reports on your topic.  Search their websites or any online libraries that they may provide.  For example, the WHO has a number of searchable online collections and the World Bank now makes all of their publications openly available online.
  • Contact known researchers in the field to determine if there are any ongoing or unpublished studies that s/he may be aware of.

Image credit:  Stack 'em high, by Antony Theobald via,, CC BY-NC-ND 2.0

Guidance documents for gray literature searching

Databases of grey literature

Sources for searching theses and dissertations

Resources for Clinical Trials