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6.4.9  Language, date and document format restrictions

Research related to identifying trials has recently focused on the effect of excluding versus including from meta-analyses trials reported in languages other than English. This question is particularly important because the identification and translation of, or at least data extraction from, trials reported in languages other than English can substantially add to the costs of a review and the time taken to complete it. For further discussion of these issues, see Chapter 10 (Section Whenever possible review authors should attempt to identify and assess for eligibility all possibly relevant reports of trials irrespective of language of publication. No language restrictions should be included in the search strategy. Date restrictions should be applied only if it is known that relevant studies could only have been reported during a specific time period, for example if the intervention was only available after a certain time point. Format restrictions such as excluding letters are not recommended because letters may contain important additional information relating to an earlier trial report or new information about a trial not reported elsewhere.