Finding studies relevant to your question should not depend solely on database searching: Other search techniques are recommended in order to avoid different forms of bias in what studies are ultimately included in the review.
Examples of other search techniques:
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.
Summarized evidence on searching and documenting approaches to grey literature:
Severn M, Mierzwinski-Urban M, Farrah K, Walter M, Spry C, Argáez C. Grey literature. Last updated 15 March 2023. In: SuRe Info: Summarized Research in Information Retrieval for HTA. Available from: https://www.sure-info.org//grey-literature
Resources to help you identify grey literature:
Grey Matters, produced by the Canadian Agency for Drugs and Technologies in Health (CADTH)
Resources to help you identify grey literature for interventions in crime & justice, education, international development, and social welfare:
Resources to help you identify Canadian health policy:
For more information on searching the grey literature, the University of Toronto Gerstein Science Information Centre has a comprehensive guide on searching the grey literature, including lists of potential resources to search.
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 as well.
Preprint servers are free online archives that allow researchers to identify studies in rapidly developing fields and can be used to identify studies that have not gone through the more formalized and traditional peer review process. Depending on the site, searching and exporting functionality may be more or less available.
Hoy, M. B. (2020, Jan-Mar). Rise of the Rxivs: How preprint servers are changing the publishing process. Medical Reference Services Quarterly, 39(1), 84-89.
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.
ISRCTN Registry - "primary clinical trial registry recognised by WHO and ICMJE that accepts all clinical research studies (whether proposed, ongoing or completed), providing content validation and curation and the unique identification number necessary for publication"
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. doi:10.1016/j.jclinepi.2017.11.003
Conference proceedings are, technically speaking, collections of conference abstracts, papers, or presentations. They are usually published in a book or journal format or available online through conference websites.
Some databases index conference abstracts, papers, and presentations at the individual level (e.g., Conference Proceedings Citation Index via Web of Science Core Collection, Embase+Embase Classic via Ovid, APA PsycInfo via Ovid, Scopus), whereas others may only index proceedings as a whole/collection/compilation (e.g., MEDLINE via Ovid or PubMed).
Conference documents can be confusing to researchers who might expect to be able to access the full-text of a paper when in fact it is often the case in health sciences that only an abstract is available. It may be necessary to contact the authors for more information.
Google Scholar can be used to identify grey literature, but there are a few issues to keep in mind:
 Haddaway NR, Collins AM, Coughlin D, Kirk S. The role of Google Scholar in evidence reviews and its applicability to grey literature searching. PLoS One. 2015;10(9):e0138237. doi:10.1371/journal.pone.0138237
 Russell DM. Advanced search operators [Internet]. 2022 [cited 2023 Jan 16].
 Harzing AW. Using Publish or Perish to do a literature review [Internet]. 2017 [cited 2021 Sep 3].
 Adams D. Exporting your data [Internet]. 2016 [updated 2021 Mar 22; cited 2021 Sep 3].
 Hickner, A. Tip #1: Bulk export from Google Scholar [Internet]. 2022 [updated 2022 Oct 10; cited 2022 Dec 7].
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 thorough, 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.
1. Reference list searching
2. Checking included studies in other relevant knowledge syntheses
3. Similar articles feature
4. Co-cited article searching
5. Forward citation searching
Forward citation searching is available as a feature in the following resources:
Literature mapping includes tools that allow you to visualize citation networks/connections/associations as well as tools that use artificial intelligence/machine learning to identify similar articles. These tools are useful if you have one or a few seed articles to work with, or if you would like to map included studies to additional citations that may have been missed by other search methods, for example.
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
Bonato S. Searching the Grey Literature: A Handbook for Searching Reports, Working Papers, and Other Unpublished Research. Lanham, Maryland: Rowman & Littlefield; 2018.
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. Systematic Reviews, 6(1), 234. doi:10.1186/s13643-017-0625-1
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