What is the weighted test?
It is often tempting to change a trial design after an interim analysis. Bauer and Khne (1994) have shown that such changes can be done while controlling the type I error, that is the risk of falsely concluding a treatment effect. Their beautiful idea is based on weighting together p-values from the two (or more) stages of the trial. What are flexible designs used for? Using the weighted test idea, researchers such as Mller and Schfer have opened up the possibility to make your design very flexible. Almost any design modification can be considered in response to interim data. They say that you could change the study population, the primary variable, the treatment duration, the doses, etc. The greatest interest has been in sample size reestimation. It is stated several times throughout the paper that flexible designs based on weighted tests are not valid. In general, how do you define the validity of a statistical approach? When we say “not valid” we mean “not providing correct statisti
