What is Grey Literature?
Examples of Grey literature include: conference abstracts, presentations, proceedings; regulatory data; unpublished trial data; government publications; reports (such as white papers, working papers, internal documentation); dissertations/theses; patents; and policies & procedures.
The Twelfth International Conference on Grey literature in Prague in 2010 arrived at the following definition:
"Grey literature stands for manifold document types produced on all levels of government, academics, business and industry in print and electronic formats that are protected by intellectual property rights, of sufficient quality to be collected and preserved by libraries and institutional repositories, but not controlled by commercial publishers; i.e. where publishing is not the primary activity of the producing body."
(Adapted from https://guides.mclibrary.duke.edu/sysreview/greylit)
Why is it important?
It is important to look outside of information published in databases in order to reduce publication bias where "results from significant positive findings are more likely to be submitted and accepted for publication." Several manuals including the Cochrane Handbook and PRISMA for systematic review protocol recommend and incorporate the use of grey literature searching.