This Subject Guide is aimed at researchers in Health, preparing literature or systematic reviews. It is now considered that for a review to have the most comprehensive and strongest base of evidence it must include grey literature.
Grey literature is an often misunderstood concept by researchers. This guide provides a greater understanding of how 'grey literature' is defined, methods for searching the grey literature, links to various primary sources, grey literature databases, critical appraisal tools and where to find further help.
The term "grey literature" refers to information that is either unpublished or not published commercially. A widely accepted definition of grey literature is "That which is produced on all levels of government, academics, business and industry in print and electronic formats, but which is not controlled by commercial publishers."1 Some examples of grey literature include:
Grey literature contrasts with "black literature", which consists predominantly of peer-reviewed publications searchable in commercial databases.
A literature search that accesses only black' literature will likely miss key information. In fact, the most prestigious evidence-based research organisations, including the Cochrane Collaboration require that, in addition to black literature, searches for grey and unpublished literature must be conducted by their reviewers in order to avoid publication bias.2
Higgins JPT, Green S (editors). Cochrane Handbook for Systematic Reviews of Interventions Version 5.1.0 [updated March 2011]. The Cochrane Collaboration, 2011. Available from www.cochrane-handbook.org.