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ContextAllocation on the basis of randomization rather than patient choice is the gold standard of unbiased estimates of efficacy in clinical medicine. However, randomly allocating patients to treatments that do not accord with their preferences may influence internal and external validity.ObjectiveTo determine whether preferences affect recruitment to trials (external validity) and outcomes in trials (internal validity)Data SourcesWe searched MEDLINE, EMBASE, PsycINFO, CINAHL, AMED, and the Cochrane Library for articles published between 1966 and September 2004. We also hand-searched several major medical journals, searched reference lists of relevant articles, and contacted authors of published preference designs. The 2 themes in the first filter of the search strategy were preferences and possible determinants of preferences.Study SelectionComprehensive cohorts and 2-stage trials that measured or …