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") Many thanks for you help! Jokel [[alternative HTML version deleted]]
On 24.06.2012 17:41, 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")Good luck this is an open source software where you can easily review the code and check if it is right. If it is not, I'd propose to send a patch to the package maintainer. Best, Uwe Ligges> Many thanks for you help! > Jokel > > [[alternative HTML version deleted]] > > ______________________________________________ > R-help at r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. >
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