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The optimal search strategy for specifically identifying reports of adverse effects has yet to be established (Golder 2006). Two main approaches can be used: using index terms and free-text searching. Both of these have limitations; it is advisable to combine them to maximize sensitivity (the likelihood of not missing studies that might be relevant). The development of a search strategy is likely to require several iterations. For instance, it may be necessary to repeat the electronic search incorporating additional index terms, subheadings and free-text terms derived from the terms used to index and describe the studies initially identified as relevant. In deciding which combination of terms to use, authors will need to balance comprehensiveness (sensitivity) against precision. Some considerations in the use of index terms and free text terms follow.