compromised power [Power / Sample Size]

posted by Helmut Homepage – Vienna, Austria, 2017-08-03 16:04  – Posting: # 17655
Views: 12,543

Hi Darya,

» The sample size by the formula I used:
» n=28

To keep everything equal you should convert the standard deviation to the CV. Hence,

library(PowerTOST)
sampleN.TOST(CV=se2CV(0.3))

+++++++++++ Equivalence test - TOST +++++++++++
            Sample size estimation
-----------------------------------------------
Study design:  2x2 crossover
log-transformed data (multiplicative model)

alpha = 0.05, target power = 0.8
BE margins = 0.8 ... 1.25
True ratio = 0.95,  CV = 0.3068783

Sample size (total)
 n     power
42   0.818541


Of course higher than the 28 by the dubious formula. As David mentioned above (quoting Zhang’s paper) power is compromised. Let’s check how much:

power.TOST(CV=se2CV(0.3), n=28)
[1] 0.6250166

Oops!

Cheers,
Helmut Schütz
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