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

 From Michael McCulloch To statalist@hsphsun2.harvard.edu Subject Re: st: RE: standard deviation for the difference in means Date Sun, 6 Jun 2010 18:07:42 -0700

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.
```
________________________________________
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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/
```