Thank you very much for that. What is then if I have unpaired and unbalanced samples?
Best regards, Cheba 2010/5/7 Bert Gunter <gunter.ber...@gene.com> > Perhaps this might help clarify: > > sample A: 10 15 20 > sample B: 12 15 22 > > Median of sample A = 15; median of sample B = 15. Sample medians are =. > But: B-A differences are 2,0,2 with a median of 2. So the median difference > does not equal the difference of the medians. > > Clarity in what you wish to test (and why!) is essential to determine how. > > > Bert Gunter > Genentech Nonclinical Statistics > > -----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 12:46 PM > To: Joris Meys > Cc: R-help@r-project.org; Thomas Lumley > Subject: Re: [R] median of two groups > > Hi all, > > Thank you for your reply. > > if done properly! What does this mean? The R-code I have is using the > R-function sample without replacement. Am I doing this properly? > > median of the differences is zero! Does this mean if I run 1000 permutation > and for each permutation I compute the median difference and as a result I > have 1000 differences. Is the the H0: median(1000 differences) =0? If yes, > which conclusion one would have from this H0? > > Best wishes, > Cheba > > > > 2010/5/7 Joris Meys <jorism...@gmail.com> > > > depends on how you interprete "absolute median difference". Is that the > > absolute difference of the medians, or the median of the absolute > > differences. Probably the latter one, so you would be right. If it's the > > former one, then it is testing whether the difference of the medians is > > zero. > > > > Cheers > > Joris > > > > > > On Fri, May 7, 2010 at 6:52 PM, Thomas Lumley > <tlum...@u.washington.edu>wrote: > > > >> On Fri, 7 May 2010, cheba meier wrote: > >> > >> 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? > >>> > >> > >> No, that is testing whether the median of the differences is zero. This > >> is not the same as testing whether the difference of the medians is > zero. > >> > >> -thomas > >> > >> > >> > >> 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. > >>> > >>> > >> Thomas Lumley Assoc. Professor, Biostatistics > >> tlum...@u.washington.edu University of Washington, Seattle > >> > >> ______________________________________________ > >> 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. > >> > > > > > > > > -- > > Joris Meys > > Statistical Consultant > > > > Ghent University > > Faculty of Bioscience Engineering > > Department of Applied mathematics, biometrics and process control > > > > Coupure Links 653 > > B-9000 Gent > > > > tel : +32 9 264 59 87 > > joris.m...@ugent.be > > ------------------------------- > > Disclaimer : http://helpdesk.ugent.be/e-maildisclaimer.php > > > > [[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. > > [[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.
