The Fourth International Conference on Grey Literature (GL '99) in Washington, DC, in October 1999 defined grey literature as follows: "That which is produced on all levels of government, academics, business and industry in print and electronic formats, but which is not controlled by commercial publishers."
Some examples of grey literature include:
There are a few reasons why researchers might be interested in accessing grey literature:
Research takes a long time to publish. Some types of grey literature can be published more quickly.
If you think about when COVID-19 first hit, quality information was hard to come by. Instead, researchers had to look at reports and data sets rather than polished, peer-reviewed journal articles.
Researchers and publishers sometimes do not publish results from studies that show negative or null results. This is not just a problem of publishers refusing to publish reports, but can also be a self-imposed censorship by researchers who do not wish to showcase unsuccessful trials. The problem with this is that journal publications are how researchers communicate findings to one another. NOT publishing unsuccessful trials means you are missing a chunk of the research story, and you might not know that a therapy is unsuccessful if people don't publish those results.
Grey literature is important because it helps to fill this evidence gap. By searching for information that is not published, you can find information that you wouldn't necessarily find in commercial publications due to publication bias.
Finding grey literature can be difficult; if you are adding a grey literature component to your search strategy, it is a good idea to start by identifying what type of research you are looking for, and then where you might find it. What other organizations might be asking similar questions? Some
You can read more about searching for Grey Literature by checking out UBC's Guide.