In 1997, The Luxembourg Convention on Grey Literature defined it as “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” (Farace 1998)
Synonyms: Fugitive literature, gray literature
Forms of Grey Lit include: Reports, Conference Abstracts, Dissertations & Theses, Registered Clinical Trials, Interviews, Patents, Newsletters, White Papers, Book Chapters
Bottom line ⇒ You are not doing a real systematic review if you don't include some gray literature searching.
“Publication bias is the term for what occurs whenever the research that appears in the published literature is systematically unrepresentative of the population of completed studies.” (Rothstein, 2005)
Systematic reviews aim to include ALL high quality studies about the research question. This is difficult to accomplish, but a missed study or studies may affect results and conclusions.
As the Cochrane Collaboration reports, studies that report statistically significant 'positive' results are:
Because of this, negative studies, equivocal results studies, and non-English studies are less likely to get published, less likely to get into the top journals, and less likely to get cited.
Not everything gets published in peer-reviewed journals, particularly English peer-reviewed journals.
Rothstein H, Sutton AJ, & Borenstein M. (2005). Publication bias in meta-analysis prevention, assessment and adjustments. Chichester, England; Hoboken, NJ: Wiley.
Dissertations and Theses aren't always easy to locate but they can be valuable resources. Search options include:
There are a number of sources for conference/meeting abstracts.
Many, but not all, literature databases also index these and allow you to include or exclude them from your search.
Additionally, most associations and societies provide access to their conference proceedings on their websites and/or in their primary journal, usually as a supplement issue. Ones relevant to your topic can be searched directly.