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:
In addition to the grey literature resources listed below, there are related library guides on: Finding Theses and Dissertations and Government Information (Health). The Public Health library guide has a page on grey literature with additional sources to search as well.
Some library databases contain grey literature in addition to journal articles. For example, Web of Science and Embase contain conference proceedings, while 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reviews of healthcare interventions, it is strongly recommended to search clinical trial registries for on-going and unpublished trials in order to limit publication bias.
Note: Since August 2019, eligible reports of RCTs or quasi-RCTs from ClinicalTrials.gov and WHO ICTRP are being identified and systematically added to Cochrane CENTRAL (other study designs registered (e.g. cohort studies) are not included).
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."