Grey literature refers to both published and unpublished research material that is not available commercially. A systematic review can be biased when it fails to report crucial information that may be hidden in some grey literature. A search of grey literature is one way to address potentially biased reporting of research results in published material.
Some examples of grey literature are:
- conference papers/conference proceedings
- clinical trials
- fact sheets, bulletins
- government documents
- informal communication (e.g. blogs, podcasts, email)
Grey literature can be the best source of up-to-date research on some topics note however that grey literature is usually not subject to peer review and must be evaluated accordingly.