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
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Thomas Lumley Assoc. Professor, Biostatistics
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______________________________________________
R-help@r-project.org mailing list
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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
______________________________________________
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PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
