## Data for 2nd stage of Potvin’s designs [Two-Stage / GS Designs]

Hi BEproff,

» 1) is it correct that GMR and CV are required for sample size calculation for 2nd stage in all Potvin's designs?

All calculation of sample size requires that CV and GMR be plugged in. You have a choice between using the observed GMR or a fixed GRM like 0.95. There is a lot of confusion about it, but the short version is: Two stage methods in all known forms only behave well if the true GMR is controlled. Therefore methods using GMR=0.95 (Potvin B and C and more) behave well when that criterion is true. Performance easily become abysmally bad if you are not in control of the GMR.

» 2) Why Method C is considered "better" for sponsors than Method B?

It is in the eye of the beholder. B has a bit lower observed alpha inflation than C. Asking why an authority does prefers C to B, or why an airline passenger prefers beef over chicken, is not productive.

» The 1st question arised because Potvin's article dated 2007 says that only CV is needed for A and B while Method C uses both GMR and CV.

Both use GMR=0.95 regardless of observed GMR.

Yes, all this is bloody confusing and it is easy to take the wrong decisions in this area. If you just remember the sentence in red above and base your decisions on it, you'l be more or less fine.

This week's list of things I absolutely detest: Corona virus, the which function in R, WIA-WIA interfaces for scanning under Windows 10, the Bee Gees, the smell of my fridge.

Best regards,
ElMaestro