Bioequivalence and Bioavailability Forum

Hello Tina,

❝ For Potvin B method for calculation, should the expectation also be 90 to 110?

GMR=0.95 for planning of stage 2.

❝ Stage II sample size is calculated when power is less than 80%. Why do we say that the study has failed at the completion of stage I if the power is more than or equal to 80%?

First of all, that's just how the authors defined the method.
Second, imagine that you have N₁=24 at the first stage but you aren't BE yet. Now you derive power and find out it is larger than 80%, and if you want to achieve just above 80% then you need N₁=20. It means you should not include any more subjects; from that perspective it would make no sense not to stop. What else could you realistically do?

Finally, the whole business with power of the Potvin methods is very funny, if not funky or even freaky. To derive power and find a sample size for stage 2 you must identify the CV or variance. That variance is conditional on and associated with the GMR point estimate. It is the treatment effects (the GMR so to say) that determines the CV, not the other way around. Then that GMR point estimate is immediately disregarded: you just assume it is 0.95 with Potvin B and C, but you keep and use the CV that was conditional on the GMR being something else. Highly weird indeed, but at least it provides some scientist with an opportunity to wonder, speculate and do their own research. There is an enormous need for improved methods in this area. We have not seen the last papers in this area - at least that's why my crystal ball says.