What is grey literature?
Grey literature is the term used to describe literature that is not formally published as a book or journal article (Higgins, 2011), including:
Related library guides:
Some library databases contain grey literature in addition to journal articles. For example, Web of Science, Embase and Medline/PubMed contain some conference proceedings; PsycINFO contains some dissertations. Since library databases do not generally provide comprehensive coverage of grey literature, other resources should be searched to identify literature that is not formally published as well.
For quantitative systematic reviews of healthcare interventions, it is strongly recommended to search clinical trial registries for on-going and unpublished trials in order to limit publication bias.
Does the inclusion of grey literature influence estimates of intervention effectiveness reported in meta-analyses?
"The exclusion of grey literature from meta-analyses can lead to exaggerated estimates of intervention effectiveness. In general, meta-analysts should attempt to identify, retrieve, and include all reports, grey and published, that meet predefined inclusion criteria."