Flexible design of two-stage adaptive procedures for phase III clinical trials

The recent popularity of two-stage adaptive designs has fueled a number of proposals for their use in phase III clinical trials. Many of these designs assign certain restrictive functional forms to the design elements of stage 2, such as sample size, critical value and conditional power functions. We propose a more flexible method of design without imposing any particular functional forms on these design elements. Our methodology permits specification of a design based on either conditional or unconditional characteristics, and allows accommodation of sample size limit. Furthermore, we show how to compute the P value, confidence interval and a reasonable point estimate for any design that can be placed under the proposed framework.