ABSTRACT
Interim analyses in group sequential trials allow making conclusions on efficacy and safety before the planned end of the trial is reached. The practical use of group sequential designs, however, is limited since they do not allow for data-driven sample size reassessments or other design changes. The distinctive feature of classical methods in group sequential analyses is that they require a data independent choice of the group sample sizes and that external factors only may influence the change or modification of the design. On the other hand, redesigning a study based on the results of an interim analysis looks attractive. It is desirable to adjust or reassess, for example, sample size calculations that are merely based on facts that are available prior to the study.
