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st: RE: standard deviation for the difference in means


From   "Lachenbruch, Peter" <Peter.Lachenbruch@oregonstate.edu>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   st: RE: standard deviation for the difference in means
Date   Sun, 6 Jun 2010 13:17:44 -0700

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

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