Futility and conditional power

In addition to looking for overwhelming efficacy there is also the possibility of stopping the trial for futility at an interim stage. It may be, for example, that the analysis of the interim data is not at all in favour of the test treatment and were the trial to continue there would simply be no real possibility of obtaining a 

CH14 INTERIM ANALYSIS AND DATA MONITORING COMMITTEES 

This method of calculating the conditional power assumes that the observed difference between the treatments at the interim stage is the true difference, termed the conditional power under the current trend. It is also possible to calculate conditional power under other assumptions, for example, that the true treatment difference in the remaining part of the trial following the interim analysis is equal to d. These calculations under different assumptions about how the future data should behave will provide a broad basis on which to make judgements about terminating the trial for futility. 

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