GREY LITERATURE BIAS:
A bias relevant to studies of knowledge synthesis (e.g. systematic review), that occurs when the results reported in published journal articles are systematically different from those described in the Grey Literature1. Grey Literature is defined as any reports, studies, essays, or other technical documents that are not specifically aimed for conventional publication, may not be peer-reviewed, and/or have restricted distribution so they are not commonly found in bibliographic retrieval systems2. Specific examples of Grey Literature include corporate documents, government documents, working papers, abstracts (summaries) of scientific papers presented at conferences, student papers or theses.
Grey Literature Bias may be classified as a specific type of Publication Bias, primarily relevant to studies of knowledge or evidence synthesis. Even though Grey Literature may not be found in peer-reviewed sources, it still may contain valid and relevant scientific findings2, and thus should be obtained and evaluated if it is applicable for the research question during a knowledge summary. If one believes that Grey Literature Bias may not be an issue for a specific research context (e.g. it is unlikely that a randomized clinical trial of a new drug would be found exclusively in the Grey Literature), then a review of the Grey Literature may not be warranted. Research scenarios when an evaluation of the Grey Literature may be necessary, is when the research question cannot be evaluated with an experimental design (e.g. the relationship between illegal drug use and miscarriages in pregnant women etc.). Grey Literature Bias may be considered a subtype of Dissemination Bias. Also see: Publication Bias, Selection Bias, and Dissemination Bias.
References:
1. Song F, Parekh S, Hooper L, Loke YK, Ryder J, Sutton AJ, et al. Dissemination and publication of research findings: an updated review of related biases. Health Technol Assess. 2010;14(8):iii, ix-xi, 1-193. (Link to Reference)
2. Porta M, ed. A Dictionary of Epidemiology. Sixth ed. New York, NY: Oxford University Press 2014. (Link to Reference)