On Jun 24, 2012, at 11:41 AM, Jokel Meyer wrote:

Dear R experts,

I have conducted a power calculation in order to estimate the number of
subjects needed to detect an effect size of d=0.28 (cohen's d) for a
difference between two independent groups (alpha level should be 0.05 and
the effect should be detected with 80% probability).
The results from the code below indicates that I would need n=400 subjects (200 in each group). This is seems so incredibly high that I mistrust my
results & wanted to ask whether I miscalculated n?

library(pwr)
pwr .t .test (d = 0.28 ,sig.level=0.05,power=0.8,type="two.sample",alternative="two.sided")

Your reaction is typical for inexperienced researchers who conduct power analyses. An effect size of .28 is considered a "small effect size" so it should not seem unreasonable that you would need 200 per group. As a quick check I Googled for an online sample size calculator and the first one I used reported the need for 202 per group for a two sided test at those levels.

--

David Winsemius, MD
West Hartford, CT

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