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Searching is a critical part of conducting the systematic review as it provides the evidence base for your research. Incomplete searches leave you open to biased results.If you are doing a full systematic review, your search strategy needs to be:
COMPREHENSIVE AND UNBIASED - This means you need to search across a number of databases and grey literature sources, and consider hand-searching bibliographies and key journals to ensure you have found as many studies as possible.
CLEAR AND REPRODUCIBLE - Which means you will need to document the terms you are using, how you combine them and where and when you have searched so that others can update and evaluate your review.
Yale University Library has some really useful videos about systematically searching the literature: Systematic Searches.