Incorporation of a stopping criterion for futility into the design of a clinical trial with one interim analysis

We examine the efficiency of clinical trial designs with one interim analysis and prespecified overall type I and II error probabilities. The designs incorporate a stopping criterion for a positive result as well as one for having interim data which indicate that continuation of the trial is futile. The classical Pocock or O'Brien-Fleming group sequential method is applied for early stopping with a positive result. The other stopping criterion is based on the idea of stochastic curtailment: given the interim data, the conditional power is calculated under an optimistic but plausible alternative; if this conditional power is less than a predetermined value, the trial is stopped due to futility.