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

 From Richard Goldstein 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:
>
>> 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
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