I’m sick of “100.0% power” [Power / Sample Size]

posted by Helmut Homepage – Vienna, Austria, 2015-07-02 01:18  – Posting: # 15010
Views: 5,520


first of all: I agree with what ElMaestro wrote…

» […] in many cases, post hoc power is more than 100%

Implying a negative (!) producer’s risk, since β = 1 – power? :-D

» or nearly 100%. It is like forced bio-equivalence.

“Forced BE” is an unofficial term describing large sample sizes where an extremely “bad” T/R-ratio is assumed beforehand and an attempt is made to “squeeze” the confidence interval to within the acceptance range. Can be an issue for the ethics committee. The EC has to deal with the risk for the subjects in the study. If I would be a member of an EC and know of a couple of studies with 24–36 subjects (same drug/re­fe­rence), I would for sure raise my hand against a protocol suggesting 120.
Post hoc power is completely irrelevant. Stop calculating it!

On another note are you sure the ~100% power are correct? I stopped counting reports on my desk where the number was just wrong. Completely wrong. Sorry to say, but some CROs are notorious for that. Example (one of the most renowned [image] CROs, study report hot off the press, June 2015). 2×2 crossover, one PK metric failed: 90% CI 112.44–127.73%, n=71 (36/35), CVintra 23.08%, “power” reported with 100.0%. Soft­ware SAS, code not given but obviously perfectly wrong – or as ElMaestro once called it:

Garbage in, garbage out.
It’s very very simple.

If power would really have been 100% (producer’s risk zero!), why the hell did the study fail? The correct (again and again and again: irrelevant!) post hoc power for the TOST is just 29%. I assume (!) that the CRO estimated “the power of the t-test to show a difference of 20%”. This was coined the “80/20 rule” by the FDA in the 1980s… which was shown1 to be crap and rightly thrown into the statistical garbage bin decades2 ago.

» Does regulator seek justification for this?

Not that I recall. Their job is to assess the patient’s risk. If you are asked for post hoc power, tell them to study their own guidelines. None (!) requires it. As you cannot be punished for being lucky (low “power”) the same is true for wasting money (very high “power”). However, in almost all cases I have seen it’s a plain miscalculation.

  1. Schuirmann DJ. A comparison of the Two One-Sided Tests Procedure and the Power Approach for Assessing the Equivalence of Average Bioavailability. J Pharmacokin Biopharm. 1987;15(6):657–80. doi:10.1007/BF01068419.
  2. FDA/CDER. Guidance for Industry. Statistical Procedures for Bioequivalence Studies Using a Standard Two-Treatment Crossover Design. Jul 1992. [image] Internet Archive.

Helmut Schütz

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