Finding sources relevant to your question should not depend solely on database searching. Supplementary search methods are recommended in order to mitigate publication bias (the practice of publishing studies with positive effects and rejecting studies with zero or negative effects). Adding this step to your methodology also ensures that you've taken steps to go beyond the formal structure of databases, including the inclusion decisions made by their publishers.
References
Review works cited lists from similar reviews and from the articles that have met your eligibility criteria.
Cited Papers
Use Google Scholar or Web of Science to review sources that have cited the articles included in your study.
Hand Search Specific Journals
Manually browse through journals that often publish studies of interest to your review. Check journal coverage in your chosen databases to see what years might not be included in your systematic search and then hand search those issues for relevant articles. (Do not be afraid; this would usually be with electronic journal issues.)
Clinical Trials
Not all clinical trials end in publications. Clinical Trial Registries help you locate otherwise unpublished or incomplete trials.
Dissertations & Theses
Reports and conference papers
Look for conference proceedings from relevant conferences and associations. Scan publication lists from related research centers and institutes.
Gray Literature
Though Google and Google Scholar searches will not be part of your systematic methods, Google can be used to find publications from organizations, centers and institutes of interest. Google Scholar may help turn up additional gray literature (see Document Types in Grey Literature) and some journal articles that may have been missed in the systematic search. See also:
Google Scholar
Google Scholar is also a valuable tool for finding articles that are too new or otherwise not indexed in your chosen databases. Use a more simplified version of your search strategy and comb the first few pages of results. You can also do a search that limits results to the last two years so that you can make sure you've captured the latest publications.
Contact Experts
Reach out to scholars that do research on your topic and solicit publications or data that may not have been formally published.
It's important to keep a clear record of your supplementary search strategies and results. Note just where and how you searched, and which included sources were found via these strategies. Take a look at this method for organizing search results. Though the example uses the database PubMed and the citation manager EndNote, the method can be translated to other databases and tools.
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