I am calculating the sample size for proportion test. I would like to have significance level =0.05, power = 0.90 and that the effect size is greater that 5%.

I would like to have statistically significance result if the difference in proportions is more that 5%.

But when I use pwr.2p.test function from pwr package to calculate sample size

pwr.2p.test(sig.level = 0.05, power =0.9, h=0.2, alternative="greater")

I have to specify effect size as Cohen's D. But it's range is said to be in (-3,3), and interpretation of this is:

The meaning of effect size varies by context, but the standard interpretation offered by Cohen (1988) is: cited from here

.8 = large (8/10 of a standard deviation unit)

.5 = moderate (1/2 of a standard deviation)

.2 = small (1/5 of a standard deviation)

My question is, how to formulate that I'd like to detect that there is more that 5% difference in proportions in 2 groups in a Cohen's d statistic?

Thanks for any help!


Are you constrained to the cohen's d approach? I've followed this site (http://ww2.coastal.edu/kingw/statistics/R-tutorials/proport.html) to do power tests recently. In my case I have a sense of the rates to expect in the samples and can plug in values for effect sizes I might encounter, so it may not work in your situation.

  • $\begingroup$ Thanks. I know what is the proportion of succes in control group and want to prove that the proportion in experimental group is at least 5% Higher :) so maybe it will work $\endgroup$ – Marcin Kosiński Nov 9 '14 at 13:51

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