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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 13:09:20 -0700 |

Thanks Michael for the simple and practical suggestion! Great books you write, by the way! Michael On Jun 7, 2010, at 12:43 PM, Michael N. Mitchell wrote:

Dear MichaelI would use the -sampsi- command and you can just plug in thevalues you have based on your published study (see -help sampsi-).Here is an example.... sampsi 10 12, sd1(5) sd2(6) n1(150) n2(150) Estimated power for two-sample comparison of means Test Ho: m1 = m2, where m1 is the mean in population 1 and m2 is the mean in population 2 Assumptions: alpha = 0.0500 (two-sided) m1 = 10 m2 = 12 sd1 = 5 sd2 = 6 sample size n1 = 150 n2 = 150 n2/n1 = 1.00 Estimated power: power = 0.8803If the mean for group1 was 10, the mean for group2 was 12, the sdfor group1 was 5, the sd for group2 was 6, the the proposed n1=150and n2=150, the power is .8803.Best luck, 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 2010-06-07 12.00 PM, Michael McCulloch wrote:Thanks Peter and Michael, What I'm wanting to do here is perform a sample size estimate with summary data obtained from a published study. 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,And, the P value based on a two-sample t test 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 inmeansbetween groups A and B? 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 Mitchell Sent: 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 themeans of two independent groups with equal population varianceswouldbe, 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 entryfor the ttest command, see -help ttest- and then see the manualentry.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.html Stata tidbit of the week - http://www.MichaelNormanMitchell.com On 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 randomly assigned 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 comparingdifferences inbetween-group means.My question is, with those values (and none other) available, isitpossible 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/* * 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/

**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>

**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 N. Mitchell" <Michael.Norman.Mitchell@gmail.com>

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