[R] median of two groups
Thomas Lumley
tlumley at u.washington.edu
Tue Apr 6 18:55:50 CEST 2010
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]]
>
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Thomas Lumley Assoc. Professor, Biostatistics
tlumley at u.washington.edu University of Washington, Seattle
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