Impact of minimum stage 2 sample size on the Type I Error [Two-Stage / GS Designs]

posted by Helmut Homepage – Vienna, Austria, 2016-12-30 01:22  – Posting: # 16908
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Dear all,

on a recent occasion… We know that the minimum n2 = 2 as required in the Q&A document is meaningless. Either a study stops in the first stage or it continues with at least two subjects anyway.

[image]However, do not go further unless you know what you are doing. If you require a minimum stage 2 sample size all studies where a smaller sample size would already be sufficient to demonstrate BE with the target power are now forced to this size. Higher sample size ⇒ more degrees of freedom ⇒ narrower CI ⇒ higher probability to pass BE.
In other words, the TIE will also increase and one would have to use a lower adjusted α.

To the right an example what would happen if one modifies Potvin’s Methods B and C at the location (n1 12, CV 20%) of the maximum TIE and naïvely applies the ‘natural constant’ α 0.0294.

Not a very good idea. Own simulations are mandatory in order to find a suitable α!

Helmut Schütz

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