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From |
Michael McCulloch <mm@pinestreetfoundation.org> |

To |
statalist@hsphsun2.harvard.edu |

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

Date |
Mon, 7 Jun 2010 12:00:27 -0700 |

Thanks Peter and Michael,

Unfortunately I do not have the raw data. I only have: 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,

Thank you for your thoughts on my question. Michael McCulloch On Jun 7, 2010, at 8:44 AM, Lachenbruch, Peter wrote:

-----Original Message-----From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Michael MitchellSent: Sunday, June 06, 2010 7:18 PM To: statalist@hsphsun2.harvard.edu Subject: Re: st: RE: standard deviation for the difference in means It is true, that the formula that Peter gave is based on the assumption of equal variances for each group. If you were using Stata, the equivalent command for comparing the means of two independent groups with equal population variances would be, for example... . sysuse auto . ttest weight, by(foreign) If you do not believe the population variances are not equal in the two groups, you could add the -unequal- option, as shown below. . ttest weight, by(foreign) unequal or you can use the -welch- option, as shown below . ttest weight, by(foreign) welch The methods and formulas are provided at the end of the manual entry for the ttest command, see -help ttest- and then see the manual entry. I hope that helps, Michael N. Mitchell Data Management Using Stata - http://www.stata.com/bookstore/dmus.html A Visual Guide to Stata Graphics - http://www.stata.com/bookstore/vgsg.htmlStata tidbit of the week - http://www.MichaelNormanMitchell.comOn Sun, Jun 6, 2010 at 6:07 PM, Michael McCulloch <mm@pinestreetfoundation.org> wrote:Thanks Peter, I'm not sure. I'm comparing two groups randomlyassigned totreatment, so they are independent, but my understanding was thepooledvariance 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 MichaelMcCulloch[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 sizeAnd, the P value based on two-sample t tests comparing differencesinbetween-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 * * 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/ * * 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/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/* * 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/ * * 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/

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/

**Follow-Ups**:**Re: st: RE: standard deviation for the difference in means***From:*"Michael N. Mitchell" <Michael.Norman.Mitchell@gmail.com>

**References**:**st: standard deviation for the difference in means***From:*Michael McCulloch <mm@pinestreetfoundation.org>

**st: RE: standard deviation for the difference in means***From:*"Lachenbruch, Peter" <Peter.Lachenbruch@oregonstate.edu>

**Re: st: RE: standard deviation for the difference in means***From:*Michael McCulloch <mm@pinestreetfoundation.org>

**Re: st: RE: standard deviation for the difference in means***From:*Michael Mitchell <Michael.Norman.Mitchell@gmail.com>

**RE: st: RE: standard deviation for the difference in means***From:*"Lachenbruch, Peter" <Peter.Lachenbruch@oregonstate.edu>

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