2.4.6.1 Grey literature

Grey literature is better defined by what it is not, than by what it is. Grey literature is essentially anything that is not controlled by the traditional, usually peer-reviewed, academic publishing market (Bonato 2018). In other words, it can include dissertations, theses, webpages, documentaries, government white papers, reports produced by leading think tanks, etc. Grey literature may also include patents, court filings, unpublished or in-process clinical trials, conference proceedings and posters. The effect of grey literature on systematic reviews may be equivocal (Godin et al. 2015; Tyndall 2016). However, prior Cochrane research shows that grey literature may affect meta-analyses on randomised trials of health care interventions (Hopewell et al. 2007). As reviewers still strive for the ideal of capturing as much literature as exists on a topic, aiming to be as comprehensive as possible, a separate grey literature search is still included in the third stage of the JBI search methodology.

Grey literature searches can include searching repositories relevant to your topic that are not indexed elsewhere, including grey literature-specific databases such as GreyMatters, GreyNet, CORE or BASE (Bielefeld Academic Search Engine). Unfortunately, many of these tools have limited sophistication in their search and exporting record capabilities. A simple keyword search or title/abstract search may be all that is available. Given these limitations, it is important to have a clear idea of what search terms and synonyms are relevant to your questions. Google and Google Scholar are useful sources of grey literature. Exporting bulk records from Google Scholar is possible using Publish or Perish (Harzig REF).

Because grey literature can be so expansive, it is best to have a clear outline ahead of time of what you will search, why and when (Bonato 2018). This can be as simple as a list of known websites to be manually checked for updates, and the date these sites were searched (Godin et al. 2015). An example template for search tracking can be found at: Research Guides: Grey Literature for Health Sciences: Getting Started