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.
