Thank you very much. These is no statistician where I am! It was clear for me from the previous email that it is possible to do permutation on the median differences even if we have unpaired and unbalanced samples. I was only confused about the null hypothesis ( median difference or difference of the medians), what Thomas and Bert wrote!
Thank you again and best regards, Cheba 2010/5/11 Greg Snow <greg.s...@imail.org> > Here is a simple example of a permutation test on medians without pairing > and different numbers of subjects in each group: > > sw <- iris$Sepal.Width[ c(101:130, 51:70) ] > group <- rep( 1:2, c(30,20) ) > > out <- replicate( 1999, { tmp <- sample(group); > median(sw[tmp==1]) - median(sw[tmp==2]) } ) > > out <- c( median(sw[group==1]) - median(sw[group==2]), out ) > hist(out) > abline(v=out[1], col='red') > mean( out >= out[1] ) > > > Hope this helps, > > -- > 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: Monday, May 10, 2010 3:02 AM > > To: Bert Gunter > > Cc: R-help@r-project.org; Thomas Lumley > > Subject: Re: [R] median of two groups > > > > 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. > [[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.