Interim analyses based on median survival times

For a typical two-arm survival study, analyses are usually performed periodically to evaluate, for example, the treatment difference. In this paper, we propose to use differences or ratios of estimated median survival times repeatedly during a study to evaluate the relative merit of two treatment groups. This new procedure provides quantitative assessments of the treatment difference and is completely model-free. Our method is illustrated with a recent aids clinical study which evaluates the efficacy of a reduced dose of AZT.