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| From | Richard Goldstein <richgold@ix.netcom.com> |
| To | statalist@hsphsun2.harvard.edu |
| Subject | Re: st: RE: standard deviation for the difference in means |
| Date | Mon, 07 Jun 2010 08:29:32 -0400 |
Michael, you have enough info to use -ttesti- (see -help ttest-) and that will give you the result you are looking for; use the "unequal" option Rich On 6/6/10 9:07 PM, Michael McCulloch wrote: > Thanks Peter, I'm not sure. I'm comparing two groups randomly assigned > to treatment, so they are independent, but my understanding was the > pooled variance assumes equal variances in each group. > > On Jun 6, 2010, at 1:17 PM, Lachenbruch, Peter wrote: > >> Are you asking for >> pooled variance: sp^2=[(n1-1)s1^2+(n2-1)s2^2]/[n1+n2-2] ? >> >> Then the sd for the difference in the means will be >> sp*sqrt(1/n1+1/n2) aka standard error. >> >> ________________________________________ >> From: owner-statalist@hsphsun2.harvard.edu >> [owner-statalist@hsphsun2.harvard.edu] On Behalf Of Michael McCulloch >> [mm@pinestreetfoundation.org] >> Sent: Sunday, June 06, 2010 12:57 PM >> To: Statalist Statalist >> Subject: st: standard deviation for the difference in means >> >> Apologies for a basic statistics question: >> >> I wish to estimate sample size for an experiment in which, from >> previously published data, I have available the following summary >> measures (but not the raw data): >> >> Group A: >> mean of a continuous variable >> standard deviation of that continuous variable >> sample size >> Group B: >> mean of the same continuous variable >> standard deviation of that continuous variable >> sample size >> And, the P value based on two-sample t tests comparing differences in >> between-group means. >> >> >> My question is, with those values (and none other) available, is it >> possible to estimate the standard deviation for the difference in >> between-group means. >> >> >> Best wishes, >> >> Michael McCulloch, LAc MPH PhD >> Pine Street Foundation >> 124 Pine Street >> San Anselmo, CA 94960-2674 >> tel: 415-407-1357 >> fax: 206-338-2391 > > Best wishes, > > Michael McCulloch, LAc MPH PhD > Pine Street Foundation > 124 Pine Street > San Anselmo, CA 94960-2674 > tel: 415-407-1357 > fax: 206-338-2391 * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/