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

posted by Helmut Homepage – Vienna, Austria, 2017-12-27 13:02 (1065 d 14:47 ago) – Posting: # 18100
Views: 27,836

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.

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Helmut Schütz
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