Developing a complex search strategy takes time and trial and error. Search strategies should begin with several articles that would likely be included in the systematic review.
Controlled vocabulary, like MeSH (Medical Subject Headings) from Medline, as well as appropriate keywords are developed and tested. Then a complex search string is created using filters that are designed to include studies meeting the criteria. The goal is to increase sensitivity in order to ensure as complete a recall as possible.
Databases: Most guidelines for medical systematic reviews require that searches be run in 2 to 3 databases, plus grey literature. Usually Medline, Embase, and Cochrane CENTRAL Register of Controlled Trials are required. However, if you are finding too few results, searching a few subject specific databases, or broadening your search strategy is required to ensure a balanced search.
Guides and Tools
Further reading:
Shojania KG, Bero LA. Taking advantage of the explosion of systematic reviews: an efficient MEDLINE search strategy. Eff Clin Pract. 2011;4(4):157-162. http://www.ncbi.nlm.nih.gov/pubmed/11525102.
Montori VM, Wilczynski NL, Morgan D, Haynes RB. Optimal search strategies for retrieving systematic reviews from Medline: analytical survey. BMJ. 2005;330(7482):68. doi:10.1136/bmj.38336.804167.47.
A well planned search protocol can help to avoid the risk of reporting bias.
Grey literature is any material that is not produced by commercial publishers, or not "systematically identified, stored or indexed".
This could include government reports, white papers, conference proceedings, newsletters, business reports, dissertation & thesis, registered trials not published yet.
Including grey literature helps to:
Knowing what resources to search and when to stop is a tricky balancing act. What resources to search depends greatly on the subject and context of your review.
For more see http://www.greynet.org/greysourceindex/documenttypes.html
Clinical Trial Resources
Regulatory Data
General Resources
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