Finding studies relevant to your question should not depend solely on database searching: Supplementary search methods are recommended in order to avoid different forms of bias in what studies are ultimately included in the review.
Examples of supplementary search methods:
Grey literature, according to the Cochrane Handbook, is usually understood to be literature not formally published in books or journals. This can include theses or dissertations, conference proceedings, clinical trials registries, white papers, government reports, and more. Some grey literature will be retrievable through database searching, but it depends on the databases you have chosen to search and what kind of content the databases index. For example, MEDLINE does not index much grey literature, whereas you can retrieve some conference proceedings indexed in Web of Science Core Collection databases available through McGill.
You may be interested in finding grey literature available on websites. One suggestion is to identify associations, organisations, institutions, etc. that are likely to make documents or reports of relevance to your question available on their websites, and to then selectively search or browse those sites.
Resources to help you identify grey literature:
Resources to help you identify grey literature for interventions in crime & justice, education, international development, and social welfare:
Theses and dissertations are potentially rich sources of grey literature given the depth of research involved in writing a thesis or dissertation. They are also usually considered grey literature in and of themselves.
Clinical trials may go unreported in the published literature. One useful method to identify unpublished clinical trials is to search clinical trials registries. The results may be available within the registries or you may need to contact the researchers associated with the trial for further information.
For more information on what and how to search for randomized controlled trials of new drugs, see:
Knelangen M, Hausner E, Metzendorf M-I, Sturtz S, Waffenschmidt S. Trial registry searches for randomized controlled trials of new drugs required registry-specific adaptation to achieve adequate sensitivity. J Clin Epidemiol. 2018;94:69-75.
Searching for citation relationships includes at least five approaches to finding references that you may not have picked up through your database searching. This can be especially important if your database search strategies were not exhaustive, if the literature on your topic is dispersed across multiple databases or subject areas, or if the vocabulary used by authors is highly heterogenous for a given idea.
Reference list searching
Included studies in other relevant knowledge syntheses
Similar articles feature
Co-cited article searching
Forward citation searching
Forward citation searching is available as a feature in the following resources:
Handsearching is a term that predates online tables of contents and generally involves reading the tables of contents of journals that are highly likely to publish literature on your topic. This may pick up studies that were missed by the database searches, for example because they used terms that you did not include in your search strategy or because the journal is not indexed in the databases you chose to search (that said, it is a good idea to search databases that index the journals relevant to your topic).
We recommend the Cochrane free online course on Hand Searching to help you find:
Belter, C. W. (2016). Citation analysis as a literature search method for systematic reviews. Journal of the Association for Information Science and Technology, 67(11), 2766-2777. Retrieved from https://onlinelibrary.wiley.com/doi/abs/10.1002/asi.23605. doi:10.1002/asi.23605
Cooper, C., Booth, A., Britten, N., & Garside, R. (2017). A comparison of results of empirical studies of supplementary search techniques and recommendations in review methodology handbooks: a methodological review. Syst Rev, 6(1), 234. doi:10.1186/s13643-017-0625-1