“There are many definitions of grey literature, but it is usually taken to mean literature that is not formally published in sources such as books or journal articles.” Cochrane Handbook- 6.2.1.8 Grey literature databases.
Formats include:
The intent of a systematic review is to synthesize all available evidence that is applicable to your research question. There is a strong bias in scientific publishing toward publishing studies that show some sort of significant effect. Meanwhile, many studies and trials that show no effect end up going unpublished. But knowing that an intervention had no effect is just as important as knowing that it did have an effect when it comes to making decisions for practice and policy-making. While not peer-reviewed, grey literature represents a valuable body of information that is critical to consider when synthesizing and evaluating all available evidence.
Trial Registers are a useful source of unpublished and ongoing trials:
Preprint is a term that tends to mean a version of a manuscript that is self-archived and shared publicly before publication in a scholarly journal - often the version prior to peer review. You may wish to include searches for preprints in your systematic review.
There are a number of different sources for finding preprints. For example:
When you are writing up your review remember to report on any grey literature searching you have done.
Describe any online or print source purposefully searched or browsed (e.g., tables of contents, print conference proceedings, web sites) and how this was done.
Examples:
Read more: PRISMA-S: An Extension to the PRISMA Statement for Reporting Literature Searches in Systematic Reviews https://doi.org/10.31219/osf.io/sfc38