Hi, you may have noticed, that your t-test and lm had not the same p-values for the difference in means, which is calculated for group2 when you use treatment contrasts and that is what R does by default (see ?contr.treatment). This is because R uses Welsh test by default. Pros and cons are beyond this post, but look at
(t1<-t.test(extra~group,data=sleep,var.equal=T)) (s1<-summary(lm(extra~group,data=sleep))) all.equal(s1$coef["group2","Pr(>|t|)"],t1$p.value) The intercept in lm is tested against 0 (one sample t-test), so the t-statistic is (mean-0)/sd, having n-k (sample size - number of parameters) degrees of freedom. cc<-s1$coef["(Intercept)",1:2] 2*(1-pt(cc[1]/cc[2],df=18)) hth. Am 16.08.2011 07:25, schrieb Lao Meng: > Hi all: > I have a question about lm on t-test. > > data(sleep) > > I wanna perform t-test to test the difference between the 2 groups: > > I can use: > t.test(extra~group) > > The t.test result shows that:t = -1.8608; mean1=0.75,mean2=2.33 > > > But I still wanna use: > summary(lm(extra~group)) > > Intercept=0.75,which is mean1,just the same as t.test. > group2=1.58 means the difference of the 2 groups,so > mean2=1.58+0.75=2.33,just the same as t.test. > And some parameters of group2(t value,Pr) are the same as t.test,since > group2 is the difference of the 2 groups. > > My question is: > How the "t value" of Intercept(group1 acturally) is calculated? > > > Thanks a lot. > > My best > > [[alternative HTML version deleted]] > > ______________________________________________ > R-help@r-project.org 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. -- Eik Vettorazzi Institut für Medizinische Biometrie und Epidemiologie Universitätsklinikum Hamburg-Eppendorf Martinistr. 52 20246 Hamburg T ++49/40/7410-58243 F ++49/40/7410-57790 ______________________________________________ R-help@r-project.org 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.
