Thanks Eik. As to your words:"The intercept in lm is tested against 0 (one sample t-test)"
So, I perform the following test: t.test(extra[group==1],mu=0) Since goup1 is regarded as reference,I do the 1-sample ttest based on group1's mean vs 0. But the result: t value= 1.3257 p-value = 0.2176 And t value and p value of s1 is: t value= 1.249 p value= 0.2276 So the t value and p value are different between 1-sample ttest of group1'mean vs 0 and s1(lm's result). What's the reason for the difference then? Thanks a lot for your help. My best. 2011/8/16 Eik Vettorazzi <e.vettora...@uke.uni-hamburg.de> > 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 > [[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.
