Bioequivalence and Bioavailability Forum

Hello Tina,

Power is the chance of showing BE if your assumptions are correct. The assumptions relate to the actual GMR and its CV (and distribution of the residual but we can forget that in this context).
If the GMR is 0.9 rather than 0.95 then you need more subjects in order to achieve a given (desired) level of certainty that the CI becomes narrow enough to meet BE.

If you are extreme un-realistic (let's say you assume GMR=1.0 but in reality some other data -perhaps from a previous study etc- suggest it is 0.5 or whatever) then the true power is exceedingly low and the study may be wasted, unethical. The IEC/IRB should capture that.

If you have confidence in 0.95 then use that, no issue at all. But if the true GMR is worse (more different from 1.0) then the true power is lower than you wish for.
Note also that the true GMR is never known. You can estimate it in a trial but it is still just an estimate. Secondly, in some cases there is no in vitro method (proper dissolution etc) that gives good info in advance about the prospective GMR.