Hello Jason, You are not specific enough. What do you mean by "significant difference"? Let's assume that indeed the incidence in A is 6% and in B is 10% and we are looking for Na and Nb such that with probability of at least 80% the mean of Nb sample from B will be at least, say, 0.03 (=3%) above the mean of Na sample from A. The solution is not unique. If Mb is the mean of the sample from B and Ma is the one from A, using Normal approximation we get the Mb is approximately normal with mean 0.10 and variance 0.1*0.9/Nb and Ma is approximately normal with mean 0.06 and variance 0.06*0.94/Na, so Mb - Ma is approximately normal with mean 0.04 and variance 0.09/Nb + 0.0564/Na. So let V be the maximal variance for which the probability that a normal rv with mean 0.04 and variance V is above 0.03 equals 0.80 (finding such V is straightforward). Then you must choose Na and Nb which satisfy 0.09/Nb + 0.0564/Na <= V. One such choice is Nb = 2*0.09/V, Na = 2*0.0564/V.
As I said, this solution is only approximate and probably not optimal, so see what other people say. Regards, Moshe. --- On Thu, 31/7/08, Iasonas Lamprianou <[EMAIL PROTECTED]> wrote: > From: Iasonas Lamprianou <[EMAIL PROTECTED]> > Subject: [R] stats question > To: r-help@r-project.org > Received: Thursday, 31 July, 2008, 2:46 PM > Dear friends, > I am not sure that this is the right place to ask, but > please feel free to suggest an alternative discussion > group. > My question is that I want to do a comparative study in > order to compare the rate of incidence in two populations. I > know that a pilot study was conducted a few weeks ago and > found 8/140 (around 6%) incidence in population A. > Population B was not sampled. Assuming this is (about) the > right proportion in the Population A what is the sample > size I need for population A and B in the main study, in > order to have power of 80% to idenitfy > significant differences? I would expect the incidence in > population B to be around 10% compared to the 6% of the > Population A. > Any suggestions? How can I do this in R? > > Jason > > > > __________________________________________________________ > Not happy with your email address?. > Get the one you really want - millions of new email > addresses available now at Yahoo! > http://uk.docs.yahoo.com/ymail/new.html > > ______________________________________________ > R-help@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. ______________________________________________ R-help@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.
