Grey literature

Grey literature in reviews

Evidence syntheses are special types of literature reviews through which researchers find evidence, appraise and combine it to form an understanding of broader research evidence on a particular question. This often informs decision-making for practitioners in the relevant field. 

Depending on the research question and review guidelines, grey literature can enhance the depth and comprehensiveness of reviews. In some reviews, particularly those in the humanities, it may form the basis of the review. It can also reduce publication bias by identifying unpublished studies.  

While there are no standard methods for including grey literature in reviews, decisions about its scope (such as document type or producer), sources to search (and when to stop searching), and screening approaches should be made during the planning stage and documented in the review protocol.

Systematic searches for grey literature

A systematic approach to grey literature searching may include: 

• grey literature databases and repositories 
• search engines (including AI search engines) 
•  known relevant websites 
• contacting authors (contact the author or organisation to request the full text when material is unavailable).

Grey literature searching, screening, and appraisal should be as transparent and well-reported as other research evidence in the review process. The PRISMA-S (PRISMA-Search) checklist provides guidance on reporting grey literature searches. 

There are fewer options for exporting results from grey literature searches into reference management software. Results may need to be exported individually to reference management software such as EndNote (for example from Google Scholar), or records may need to be created manually.

Screening grey literature in reviews

Screening (to select grey literature against selection criteria) is often conducted in the source itself rather than with screening software. For example, a decision might be made to screen a certain number of results pages from a search engine search. Decisions can be recorded using a spreadsheet. 

Relevant records should be added to a reference management software, if possible. If using EndNote, grey literature references should be added to a separately created group in preparation for full-text screening and reporting.

Evaluating grey literature in reviews

All studies included in evidence syntheses need to be evaluated, including grey literature. Whist there are few formal tools, the AACODS checklist is helpful in evaluating grey literature.

Reporting grey literature in reviews

It’s important to document and report approaches to grey literature in reviews to ensure transparency. This can include: 

• the source or search engine used 
• the search terms applied (include the full search strategy if possible) 
• the date the search was conducted 
• details of the screening methods used 
• any limiters or filters applied 
• the team member responsible for conducting the search.