The structure of a search strategy should be based on the main concepts being examined in a review. For a Cochrane review, the review title should provide these concepts and the eligibility criteria for studies to be included will further assist in the selection of appropriate subject headings and text words for the search strategy.
It is usually unnecessary, and even undesirable, to search on every aspect of the review’s clinical question (often referred to as PICO – that is Patient (or Participant or Population), Intervention, Comparison and Outcome). Although a research question may address particular populations, settings or outcomes, these concepts may not be well described in the title or abstract of an article and are often not well indexed with controlled vocabulary terms. They generally, therefore, do not lend themselves well to searching. In general databases, such as MEDLINE, a search strategy to identify studies for a Cochrane review will typically have three sets of terms: (1) terms to search for the health condition of interest, i.e. the population; (2) terms to search for the intervention(s) evaluated; and 3) terms to search for the types of study design to be included (typically a ‘filter’ for randomized trials). CENTRAL, however, aims to contain only reports with study designs possibly relevant for inclusion in Cochrane reviews, so searches of CENTRAL should not use a trials ‘filter’. Filters to identify randomized trials and controlled trials have been developed specifically for MEDLINE and guidance is also given for searching EMBASE: see Section 6.4.11 and subsections. For reviews of complex interventions, it may be necessary to adopt a different approach, for example by searching only for the population or the intervention (Khan 2001).