It looks like you're using Internet Explorer 11 or older. This website works best with modern browsers such as the latest versions of Chrome, Firefox, Safari, and Edge. If you continue with this browser, you may see unexpected results.
The following are some of the sources consulted while putting together this guide. These may also be of interest for further information on this subject.
A three-part Web conference series organized by Academy Health. Recordings of the presentations and presentations slides are available. 101: Shining a White Light on Grey Literature 102: Advanced Search Strategies for Grey Literature 103: Standing out in a Sea of Grey Literature
Scherer RW, Langenberg P, von Elm E. Full publication of results initially presented in abstracts. Cochrane Database of Systematic Reviews 2007, Issue 2. Art. No.: MR000005. DOI: 10.1002/14651858.MR000005.pub3.