The statistical development of the multisite Brief Strategic Family Therapy (BSFT) Trial of the National Institute on Drug Abuse's Clinical Trials Network provides a useful, real example of how an effectiveness trial can differ from an efficacy trial. In particular, two design elements distinguish this effectiveness trial from an efficacy trial. First, because the goal of the trial is to show that the use of BSFT would be an improvement on current practice, it was decided to compare BSFT to treatment as usual at each location. This decision ensures that the trial has the most ecological validity to the participating community treatment providers. Second, the desire to generalize the results to general clinical practice dictates that variability (in effect) across community treatment providers be estimated using a random effects model. These two decisions jointly influence the sample size calculations. Allowing variation in treatment as usual, will increase the variability in effect sizes across sites and estimation of this variability as a random effect necessitates a larger sample size (both number of community treatment providers and participants per community treatment provider), than is the case for a fixed site effect estimate. Details of these effects and their implications for the statistical design are presented.
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