Multistage test procedure for testing Blackwelder's hypothesis of equivalence

Suppose, in a clinical trial, the interest is to show that the effectiveness of an experimental therapy is no worse than that of the standard therapy by more than a specified amount, say delta units. Blackwelder (1) discussed this problem in clinical trials where the outcome of interest is dichotomous. The group sequential procedures developed by DeMets and Ware (2) are valid to test Blackwelder's hypothesis for the case delta = 0. In this paper, the asymmetric procedure of DeMets and Ware is modified to handle the case delta > 0. A two-stage procedure is considered in a drug interaction study that focuses on a specific side effect as the event of interest.