Grey literature is a catch-all term for material published by non-commercial sources such as government agencies, industry bodies, NGO's or academic institutions. It includes reports, guidelines, policy statements, technical standards, trial registries, dissertations or newsletters.
Grey literature is frequently high quality and up to date information. In some areas it may be the only information available. However it is not peer reviewed and should be evaluated with this in mind.
If you are aiming to be comprehensive in your search then grey literature should be included.
To account for publication bias. Published studies have been shown to show greater treatment effects than unpublished or grey studies (Hopewell et al, 2007). Results may also appear in grey literature such as conference proceedings sooner than in journals due to reduced publication lag. These factors are particularly important for people undertaking systematic reviews.
In novel areas where the body of formally published material is small.
in qualitative research where commentaries or advocacy positions are relevant.
Unfortunately grey literature is generally not included in databases such as Medline or Embase. Instead searches will need to target multiple sources. The following resources can help:
Trip database is a search engine for biomedical and health material. Once searched look at the right hand sidebar to restrict results to guidelines, regulatory materials, blogs etc.
NICE Evidence Search is run by the UK's National Health Service. It includes evidence summaries, patient information, material from professional bodies and more.
Google Advanced Search. Use advanced search to restrict to particular domains eg .gov or file types eg .pdf.
OAIster searches institutional repositories and digital libraries.
Grey Matters is produced by the Canadian Agency for Drugs and Technologies in Health. Grey Matters includes an extensive list of national and international HTA web sites, drug and device regulatory agencies, clinical trial registries, health economics resources and drug formulary web sites.