Permutation tests are real tests (if done properly), but one subtle but important note: The null hypothesis being tested is that the 2 distributions are identical, the medians being equal is part of that, but the null includes more than just that assumption.
-- Gregory (Greg) L. Snow Ph.D. Statistical Data Center Intermountain Healthcare greg.s...@imail.org 801.408.8111 > -----Original Message----- > From: r-help-boun...@r-project.org [mailto:r-help-boun...@r- > project.org] On Behalf Of cheba meier > Sent: Friday, May 07, 2010 6:01 AM > To: Thomas Lumley > Cc: R-help@r-project.org > Subject: Re: [R] median of two groups > > Dear Thomas, > > I have been running simulations in order me to understand this problem! > I > have found something online where the absolute median difference is > computed > and permutations are ran to compute a p-value. Is such a test (if I can > call > it a test) tests the null hypothesis that median group 1 = median group > 2? > > Thank you in advance for your help. > > Regards, > Cheba > > 2010/4/6 Thomas Lumley <tlum...@u.washington.edu> > > > > > > > None of them. > > > > - mood.test() looks promising until you read the help page and see > that it > > does not do Mood's test for equality of quantiles, it does Mood's > test for > > equality of scale parameters. > > - wilcox.test() is not a test for equal medians > > - ks.test() is not a test for equal medians. > > > > > > Mood's test for the median involves dichotomizing the data at the > pooled > > median and then doing Fisher's exact test to see if the binary > variable has > > the same mean in the two samples. > > > > median.test<-function(x,y){ > > z<-c(x,y) > > g <- rep(1:2, c(length(x),length(y))) > > m<-median(z) > > fisher.test(z<m,g)$p.value > > } > > > > Like most exact tests, it is quite conservative at small sample > sizes. > > > > -thomas > > > > > > On Tue, 6 Apr 2010, cheba meier wrote: > > > > Dear all, > >> > >> What is the right test to test whether the median of two groups are > >> statistically significant? Is it the wilcox.test, mood.test or the > >> ks.test? > >> In the text book I have got there is explanation for the Wilcoxon > (Mann > >> Whitney) test which tests ob the two variable are from the same > population > >> and also ks.test! > >> > >> Regards, > >> Cheba > >> > >> [[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. > >> > >> > > Thomas Lumley Assoc. Professor, Biostatistics > > tlum...@u.washington.edu University of Washington, Seattle > > > > > > [[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. ______________________________________________ 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.