Evaluating the adaptive performance of flexible sample size designs with treatment difference in an interval.

In clinical trials, the study sample size is often chosen to provide specific power at a single point of a treatment difference. When this treatment difference is not close to the true one, the actual power of the trial can deviate from the specified power. To address this issue, we consider obtaining a flexible sample size design that provides sufficient power and has close to the 'ideal' sample size over possible values of the true treatment difference within an interval. A performance score is proposed to assess the overall performance of these flexible sample size designs. Its application to the determination of the best solution among considered candidate sample size designs is discussed and illustrated through computer simulations. 2007 John Wiley & Sons, Ltd