Grey literature is information that is produced outside of commercial publishing channels. This includes print and electronic material produced by organisations such as governments, educational institutions, business and industry associations.
Examples of grey literature include:
Theses and dissertations, conference proceedings, newsletters, reports, government documents, informal communications, translations, census data, research reports, technical reports, standards, patents, videos, clinical trials and practice guidelines, eprints, preprints, wiki articles, emails, blogs, listserv archives, repository content.
Here are some reasons why you should consider including grey literature in your search.
Although grey literature is not usually peer reviewed, it may still be a good and reliable source of information for the review topic.
It is necessary to evaluate everything you find.