"MARK LEEDS" <[EMAIL PROTECTED]> writes: > thanks. i assumed we we were talking about the standard textbook difference > between the t test and pairwise t test. > my bad.
Notice the difference between paired and pairwise... > ----- Original Message ----- > From: "Chuck Cleland" <[EMAIL PROTECTED]> > To: "MARK LEEDS" <[EMAIL PROTECTED]> > Cc: "Li,Qinghong,ST.LOUIS,Molecular Biology" <[EMAIL PROTECTED]>; > <r-help@stat.math.ethz.ch> > Sent: Thursday, September 07, 2006 6:44 PM > Subject: Re: [R] pairwise.t.test vs. t. test > > > > MARK LEEDS wrote: > >> no, because the formula for the test statistics ( even assuming that > >> variances are equal ) of the two different tests are different. in the > >> pairwise t test, the pairwise differences are > >> viewed as one sample so it turns into a one sample test. any intro stat > >> book > >> will have the formulas. > >> > >> mark > > > > Actually, I think the difference is due to the SD being pooled across > > all 3 groups in the pairwise.t.test, but just 2 groups in t.test. > > > >> ----- Original Message ----- > >> From: "Li,Qinghong,ST.LOUIS,Molecular Biology" > >> <[EMAIL PROTECTED]> > >> To: <r-help@stat.math.ethz.ch> > >> Sent: Thursday, September 07, 2006 5:07 PM > >> Subject: [R] pairwise.t.test vs. t. test > >> > >> > >>> Hi, > >>> > >>> If I set the p.adjust="none", does it meant that the output p values > >>> from > >>> the pairwise.t.test will be the same as those from individual t.tests > >>> (set > >>> var.equal=T, alternative="t")? > >>> > >>> I actually got different p values from the two tests. See below. Is it > >>> supposed to be this way? > >>> > >>> Thanks > >>> Johnny > >>> > >>>> x > >>> [1] 61.6 52.7 61.3 65.2 62.8 63.7 64.8 58.7 44.9 57.0 64.3 55.1 50.0 > >>> 41.0 > >>> [15] 43.0 45.9 52.2 45.5 46.9 31.6 40.6 44.8 39.4 31.0 37.5 32.6 23.2 > >>> 34.6 > >>> [29] 38.3 38.1 19.5 21.2 15.8 33.3 28.6 25.8 > >>>> Grp > >>> [1] Yng Yng Yng Yng Yng Yng Yng Yng Yng Yng Yng Yng Med Med Med Med Med > >>> Med > >>> [19] Med Med Med Med Med Med Old Old Old Old Old Old Old Old Old Old Old > >>> Old > >>> Levels: Yng Med Old > >>>> pairwise.t.test(x=x,g=Grp,p.adjust.method="none") > >>> Pairwise comparisons using t tests with pooled SD > >>> > >>> data: x and Grp > >>> > >>> Yng Med > >>> Med 1.0e-06 - > >>> Old 2.0e-12 2.6e-05 > >>> > >>> P value adjustment method: none > >>> > >>> > >>>> t.test(x=x[1:12],y=x[25:36],var.equal=T, alternative="t") > >>> Two Sample t-test > >>> > >>> data: x[1:12] and x[25:36] > >>> t = 10.5986, df = 22, p-value = 4.149e-10 > >>> alternative hypothesis: true difference in means is not equal to 0 > >>> 95 percent confidence interval: > >>> 24.37106 36.22894 > >>> sample estimates: > >>> mean of x mean of y > >>> 59.34167 29.04167 > >>> > >>> [[alternative HTML version deleted]] > >>> > >>> ______________________________________________ > >>> R-help@stat.math.ethz.ch mailing list > >>> https://stat.ethz.ch/mailman/listinfo/r-help > >>> PLEASE do read the posting guide > >>> http://www.R-project.org/posting-guide.html > >>> and provide commented, minimal, self-contained, reproducible code. > >>> > >> > >> ______________________________________________ > >> R-help@stat.math.ethz.ch mailing list > >> https://stat.ethz.ch/mailman/listinfo/r-help > >> PLEASE do read the posting guide > >> http://www.R-project.org/posting-guide.html > >> and provide commented, minimal, self-contained, reproducible code. > > > > -- > > Chuck Cleland, Ph.D. > > NDRI, Inc. > > 71 West 23rd Street, 8th floor > > New York, NY 10010 > > tel: (212) 845-4495 (Tu, Th) > > tel: (732) 512-0171 (M, W, F) > > fax: (917) 438-0894 > > > > ______________________________________________ > R-help@stat.math.ethz.ch mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. > -- O__ ---- Peter Dalgaard Øster Farimagsgade 5, Entr.B c/ /'_ --- Dept. of Biostatistics PO Box 2099, 1014 Cph. K (*) \(*) -- University of Copenhagen Denmark Ph: (+45) 35327918 ~~~~~~~~~~ - ([EMAIL PROTECTED]) FAX: (+45) 35327907 ______________________________________________ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
