Grey literature searches are now commonly included in the research methods sections of literature and systematic reviews as way of demonstrating a comprehensive search for information and for addressing publication bias.  Grey literature can be obtained from a wide range of sources such as conferences, government, professional and regulatory organisations, trial registries, and various repositories.  Searching the grey literature can be a daunting task and therefore requires a strategic approach in which you search those resources that make the most sense for your research question. 

Figure 1 illustrates a strategy for locating grey literature as part of the overall research process.¹

• Step 1 and 2 of the process involve identification of suitable search terms and applying those terms in relevant databases of peer-reviewed literature (Example, MEDLINE, Scopus, etc).

• The "Hand search" referred to in Step 3, begins the grey literature search, ackowedges that database indexes will not find all relevant information and a visual scan of electronic or print (i.e. a trip to the Library) primary sources is required for a comprehensive search.

• The final part of the process, Step 4 involves applying suitable search terms to a web search engine (Example Google, Bing) and/or specialist grey literature database.

Once you have performed your grey literature search, good practice is to document the strategy used either as a narrative only, or a narrative that refers to a table of sources, search terms, limiters, search results, excluded results and number of studies included for final consideration.

References

1 . Adapted from Duffield, A. et al. (2004). "Process to Identify the Grey Literature", Review of the published literature for the impact and cost-effectiveness of six nutrition related emergency interventions, Report prepared by the ENN, p. 49.  Available: http://www.unscn.org/layout/modules/resources/files/ENN_literature_review_of_cost-effectiveness_of_6_nutrition_i.pdf