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Grey Literature

A guide for navigating additional unpublished works in comprehensive evidence syntheses (systematic reviews, scoping reviews, and integrative reviews).

Why Use Grey Literature?

Why Use Grey Literature?

While evidence syntheses require close review and comprehensive perusal of the black and white literature provided in scholarly publications, authors are also encouraged and expected to review grey literature sources.

Prevalence of Grey Lit

The Prevalence of Grey Literature

Research shows that the trends in literature citations are becoming more and more inclusive of grey literature:

  • In a study done on health technologies reports, 47% of references cited in the observed bibliographies qualified as grey literature (Farrah & Mierzwinski-Urban, 2019).
  • Additionally, in a review of 6 selected nursing journals, 10.4% of all citations included in articles were grey literature (Woods, Phillips, & Dudash, 2020).

As authors aim to be more comprehensive in their research, it is increasingly important to consider all available streams of literature.

The Standards

Consulting the Standards

Several books and standards encourage or require authors to review grey literature when conducting evidence syntheses. Because relevant research may have been conducted but not published in traditional formats, researchers should consider a strategy to include reviewing grey literature.

Pros and Cons

The Pros and Cons of Grey Literature

The following table, from Doing a Systematic Review: A Student's Guide, 2nd Edition, provides insight on the benefits (and limitations) in utilizing grey literature in an evidence synthesis (Boland, Cherry, & Dickson, p. 65).

  Published Literature Grey Literature
  • Easier to locate
  • Easier to systematically and transparently search
  • Easier to clearly report results in thesis
  • Easier to transfer results of search to reference management software
  • Usually peer-reviewed
  • Allows access to diverse evidence sources
  • Reduces risk of publication bias
  • Often not limited by restricted word count
  • Searches can identify ongoing or unpublished studies
  • Data might be limited
  • Risk of publication bias
  • Harder to locate
  • Peer-review status is often unclear, and therefore evidence may be more likely to be biased
  • Harder to search systematically and transparently
  • Harder to store results of searches
  • Harder to clearly report methods in thesis