Grey literature refers to material that is not published by academic journals and is produced outside of traditional publishing which is not formally or commercially published.
It is produced by government departments or agencies, local authorities, academic institutions, professional or scholarly associations, non-profit organisations, charities and other organisations.
It includes, but is not limited to, the following:
Grey literature can be a good source of up-to-date literature for newer areas of research but such literature is often not well represented in library databases.
Finding grey literature involves a mixed approach to searching that requires searching some databases, using search engines along with browsing websites and other materials that are relevant to the research area.
Identifying grey literature resources can be time consuming - It might involve extensive web searches for relevant websites before you start the formal search process. A good way to find grey literature is to identify organizations that might be publishing the type of information on topics that you're interested in and then to search their websites—paying attention to website sections with names like "Documents", "Reports", and "Library".
Which websites to search will be determined by the review topic and type:
It is also important to remember that Grey literature is not peer reviewed so may not be of the highest quality; whereas, the most highly regarded journals are peer reviewed. Furthermore, it may not always be possible to weigh the appropriateness of the research methods used in unpublished research. Grey literature can take all sorts of forms, care must be taken when acquiring complex data and information.
All sources of literature should be evaluated. Grey literature is usually not subject to the same peer-review process or quality checks as books, journals and other commercially published literature.
When evaluating Grey literature, consider the following criteria:
Grey literature can be very useful as very recent information, or research, may not yet have been officially published in a journal article or book - but might be available in a working paper or conference paper.
Reports from professional bodies (whose emphasis is on specific practice areas) may study certain topics in more depth, or on a more practical level, than mainstream publications.
Use Google's site limits to only search a government or institution's site or top-level domain, for instance use site:gov.ie to limit your search to Irish government sites for government services and information. This technique paired with keywords in Google is a quick way to locate grey literature. Example search: site:gov.ie finance.
There is some grey literature in Google Scholar — mainly theses and dissertations in institutional repositories.
Search for studies should be as extensive as possible in order to reduce the risk of reporting bias and to identify as much relevant evidence as possible - from the Cochrane Handbook for Systematic Reviews of Interventions.