There are a minimum of four kinds of searching that go into a systematic review.
Preliminary Search – This is the kind of searching most people do when they start thinking about conducting a systematic review. The results from a preliminary search are not exhaustive, and should not be used as the sole source of data for your systematic review. The goals of the preliminary search include: identifying existing reviews, assessing volume of potentially relevant studies (assume an exhaustive search will identify about 2-3 times the number located in a preliminary search), and locating at least 2-5 example articles that meet your review criteria.
Exhaustive Database and Grey Literature Search – This is the search designed by a librarian trained on how to design searches for systematic reviews. One goal of an exhaustive search is to Identify all publications and as much grey literature as possible that meet study requirements. Another goal is to document and report the exhaustive search in such a way that it can be replicated for updates and reproduced by others after publication. For a librarian designed systematic review search fill out the Systematic Review Search Request form.
Hand Search - Identify grey literature like conference proceedings, abstracts for posters, and presented papers not indexed in online databases. Sources to hand search include: subject specific professional association websites, major relevant journals, bibliographies of all included studies, and bibliographies of on topic reviews. It is best for the subject experts to do the hand searching since they are most likely to have access to conference archives on professional society websites. It is important to document all sources searched by hand and what was located for reporting and creating the PRISMA Flow Diagram.
Contact Experts – The goal of contacting experts is to determine if more than one paper had been published on the same study and to identify unregistered studies with unpublished results or potential results.