The term grey literature refers to all information that isn't commercially published. This can include the following types of documents:
Including grey literature in a systematic review can strengthen the overall product by including data not found in the published literature thereby minimizing publication bias, for example, including ClinicalTrials.gov or other trial registries in your search strategy,
All sources of information, including grey literature, which is rarely peer reviewed, should undergo quality assessment and appraisal before being included in the final SR analysis.
For a more detailed explanation, see ITEM 2A: ONLINE RESOURCES of the PRISMA-S Explanation and Elaboration document.
The Grey Literature (in the Health Sciences) guide contains links to databases, search engines, and websites relevant to some of the more commonly searched types of grey/gray literature in the health sciences: policy positions, reports, briefings, evaluations of programs, white papers, clinical trials, regulatory information for drugs and medical devices, grants, theses & dissertations, conferences, documents from professional organizations, and statistics.
The value that grey literature brings to a research question varies greatly from topic to topic. As such, the decision around whether to include grey literature must be determined by each research team. See the Where to Begin? box in the Grey Literature guide for examples of elements to consider in order to get started with this process, and to establish parameters/scope in alignment with resource limitations.