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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