On Wed, 8 Aug 2007, Nair, Murlidharan T 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? >
Yes. You are probably looking for the version of the t.test that assumes equal variances (the original one), so you need var.equal=TRUE. -thomas > -----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. > Thomas Lumley Assoc. Professor, Biostatistics [EMAIL PROTECTED] University of Washington, Seattle ______________________________________________ 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.