Grey (or Gray) literature refers to research that resides in formats that are not commercially published and are not peer reviewed like technical reports, pre-prints, clinical trials, statistical reports, theses/dissertations, conference proceedings, technical standards, and more. The inclusion of grey literature is incredibly important because it reduces publication bias, another hallmark of the systematic review research method.
What does that mean?
What typically shows up in published literature is bigger studies with stronger methods and unique or positive results. Smaller studies with negative or null results are often sidelined and yet, are highly valuable in their contribution to the development of research. Conducting searches that capture these studies, make for a more comprehensive research result and the calculated effect found in your metanalysis, if you plan to include one.
There are many ways to discover grey literature. Often this information resides freely on the web and some in subscription databases (if it's a conference proceeding and you find it in a library database, it's still grey literature, for example). Also, keep vigilant as you read your articles, as they will often mention grey literature sources. The following list is not a comprehensive list; however, it will get you started.
Revisit your initial list of databases. Are there any others you should consider? This is not a comprehensive list and only contains content that is currently accessible at SU. Contact your subject librarian if you have any questions using these or accessing others that do not appear here.
Consider these and others in the Social Sciences category.
In addition to the resources listed above, consider these additional databases.