Would you be so kind answering our questions? [Power / Sample Size]

posted by Helmut Homepage – Vienna, Austria, 2017-12-27 13:02  – Posting: # 18100
Views: 18,291

Hi kms,

» » study planned for 90% power: The chance to obtain a post hoc power of ≥95% is ~35%.
» Is it a thumb rule?

No; obtained in simulations by the R-code I posted above. There is only one – rather trivial – rule of thumb: The chance to get a post hoc power which is either lower or higher than the target is ~50%.

» how it should be calculated?

Once you performed the simulations, use

cat("Simulated studies with post hoc power \u22650.95:", sprintf("%.2f%%",
    100*length(res[, 3][res[, 3] >= 0.95])/nsims), "\n")

You should get

Simulated studies with post hoc power ≥0.95: 34.97%

Adapt the relevant data according to your needs. For CV 0.30, T/R 0.90, and target power 80% you would get only 7.77% in the range 0.95–0.99 and 8.64% ≥0.95.

» Yes, on getting posthoc analysis, i found three aspects what you said:
» After experiment, it came to know that
» 1. CV getting lower
» 2. GMR close to 1
» 3. No dropouts/withdrawls (though prior consideration of 10% dropouts)

Fine. Can you explain to us why you performed a “posthoc analysis” at all? What did you want to achieve? To repeat ElMaestro:
» » » Try and ask yourself which question post-hoc power actually answers. Try and formulate it in a very specific sentence.

For the 5th time (already asked #1, #2, #3, #4): An example would help.
We tried to answer your questions. It would be nice if you answer ours as well.

Helmut Schütz

The quality of responses received is directly proportional to the quality of the question asked. ☼
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