Bingo! [Two-Stage / GS Designs]

posted by Helmut Homepage – Vienna, Austria, 2016-12-30 18:00  – Posting: # 16916
Views: 3,887

Hi ElMaestro,

now you got it!

» – the alpha2 that works for n2,min=2 or 0 or 1 or whatever is not necessarily the alpha2 that works for n2,min=1.5*n1, all other factors being equal.

Yes.

» It is tempting to say power increases with sample size, and since type I error is a kind of power, this is the logic behind the observation.

Yes.

» I think the issue is somewhat more complex than just that. These two-stage thingies are funny objects that defy all kinds of logic.

Maybe and yes.

» Does it change anything though??

For me, no.

» I mean you and I both argued in the past that universally functional alpha's do not exist, so whenever someone makes a smart/clever/sophisticated/dumb/intelligent/braindead amendment to Potvin B or C etc, then simulations should always be undertaken to make sure the type I error is not compromised.

Exactly. To quote Jones and Kenward.*

[…] before using any of the methods […], their operating characteristics should be evaluated for a range of values of n1, CV and true ratio of means that are of interest, in order to decide if the Type I error rate is controlled, the power is adequate and the potential maximum total sample size is not too great.

(my emphases)



Cheers,
Helmut Schütz
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