As Thomas Lumley noted, there exist several versions of t-test. If you use t1 <- t.test(x,y) then no assumption is made of x and y having equal variance and of the two sample sizes being equal and then an approximate t-test is used with an approximate number of degrees of freedom (and this is what you got). If you use t2 <- t.test(x,y,var.equal=TRUE) then equal variance is assumed and you get 8 degrees of freedom. If you use t3 <- t.test(x,y,paired=TRUE) then equal sample sizes are assumed and the number of degrees of freedom is 4 (5-1).
--- "Nair, Murlidharan T" <[EMAIL PROTECTED]> wrote: > Indeed, I understand what you say. The df of freedom > for the dummy example is n1+n2-2 = 8. But when I run > the t.test I get it as 5.08, am I missing something? > > > -----Original Message----- > From: Moshe Olshansky [mailto:[EMAIL PROTECTED] > > Sent: Tuesday, August 07, 2007 9:05 PM > To: Nair, Murlidharan T; r-help@stat.math.ethz.ch > Subject: Re: [R] small sample techniques > > Hi Nair, > > If the two populations are normal the t-test gives > you > the exact result for whatever the sample size is > (the > sample size will affect the number of degrees of > freedom). > When the populations are not normal and the sample > size is large it is still OK to use t-test (because > of > the Central Limit Theorem) but this is not > necessarily > true for the small sample size. > You could use simulation to find the relevant > probabilities. > > --- "Nair, Murlidharan T" <[EMAIL PROTECTED]> wrote: > > > If my sample size is small is there a particular > > switch option that I need to use with t.test so > that > > it calculates the t ratio correctly? > > > > Here is a dummy example? > > > > รก =0.05 > > > > Mean pain reduction for A =27; B =31 and SD are > > SDA=9 SDB=12 > > > > drgA.p<-rnorm(5,27,9); > > > > drgB.p<-rnorm(5,31,12) > > > > t.test(drgA.p,drgB.p) # what do I need to give as > > additional parameter here? > > > > > > > > I can do it manually but was looking for a switch > > option that I need to specify for t.test. > > > > > > > > Thanks ../Murli > > > > > > > > > > [[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.
