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!
Best Answer
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.