Grey Literature is literature that is not published in the traditional sense such as clinical trials data, government documents, theses, conference proceedings. It is important to include grey literature in your review in order to broaden its scope, reduce publication bias, and include emerging research.
We are often asked what database to search for grey literature. The truth is that there isn't just one and it depends on the topic. Consider your topic and its stakeholders. Checking the publications written by organizations and associations relevant to your topic is a good way to start.
Here are some starting points for grey literature searching:
Conference papers, abstracts, and proceedings are included some databases such as Embase, Scopus, and Conference Proceedings Citation Index (part of Web of Science Core Collection), available through subscription.
You can also search individual conference proceedings to locate relevant papers.
Researchers may decide to search Google Scholar for studies but may sometimes limit to a certain number of results such as the first 100 or 200.
Google Scholar searches may retrieve grey literature not indexed in conventional academic databases. Keep in mind that Google Scholar is a search engine not a bibliographic database.
Google Scholar has an “Advanced Search” which can help focus the vast number of results. Click on the menu icon in the top left corner to access. See also Google Advanced Power Search Quick Reference.