Grey literature is "Information produced on all levels of government, academics, business and industry in electronic and print formats not controlled by commercial publishing i.e. where publishing is not the primary activity of the producing body."
- As defined by the Grey Literature Network Service, 2004
Simply put, Grey Literature consists of materials that are unpublished or have been published outside of the typical commercial or academic publishing environment.
It is often more difficult to find and obtain than the published literature, and may include such things as conference procedings, policy studies and reports, white papers, annual reports, draft legislation, think-tank reports.
Grey literature can be used in a wide variety of ways, depending on the type. Most offer data and results which can supplement original research. Many detail policies or procedures which can be used to improve existing methods.
For the most part, grey literature can be used in the same way as scholarly literature: by setting a foundation on which other types of research can be conducted.
Examples of grey literature include:
Gray literature can be an important source of data and information. Though not published in the traditional academic outlets, it is produced by researchers in the field. It can be made available more quickly and without the rigid format of academic publishing. Gray literature can also offer greater detail than other types of literature.
In addition, it can reduce positive publication bias - negative results are often reported in the grey literature but not in published work.
Within scientific publishing, the outcome of a paper can often affect the decision by a journal or authority to publish it. Publishers often show a strong bias towards publishing studies which show some sort of significant effect over studies which do not show the expected outcome. This is known as publication bias.
Publication bias can have a serious impact on the existing literature, since studies with and without significant findings are usually conducted in the same manner. Since studies without significant findings are much less likely to get published, it reduces the impact that the study could have on a given discipline. However, knowing that an intervention had no effect is just as important as knowing that it did have an effect when it comes to making decisions for practice and policy-making.
Thus, grey literature can be critical when conducting meta-analyses.