That's a robust way of obtaining a p-value, and can be classified as a test.
The important trick here is to take into account that you have two tails in
a distribution. If the p-value is calculated taking both tails into account,
then it indeed tests the null hypothesis that median group1 - median group2
= 0 (which is equivalent to median group1 = median group2).
Cheers
Joris
On Fri, May 7, 2010 at 2:00 PM, cheba meier <cheba.me...@googlemail.com>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?
>
> 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
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> PLEASE do read the posting guide
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> 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
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