Re: st: RE: standard deviation for the difference in means

[Michael McCulloch <mm@pinestreetfoundation.org>]

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
>
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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/