[R] Power calculation using pwr.t.test()

[David Winsemius]

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