[R] median of two groups
tlumley at u.washington.edu
Fri May 7 18:52:19 CEST 2010
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.
> Thank you in advance for your help.
> 2010/4/6 Thomas Lumley <tlumley at 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.
>> g <- rep(1:2, c(length(x),length(y)))
>> Like most exact tests, it is quite conservative at small sample sizes.
>> 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
>>> 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!
>>> [[alternative HTML version deleted]]
>>> R-help at r-project.org mailing list
>>> PLEASE do read the posting guide
>>> and provide commented, minimal, self-contained, reproducible code.
>> Thomas Lumley Assoc. Professor, Biostatistics
>> tlumley at u.washington.edu University of Washington, Seattle
> [[alternative HTML version deleted]]
> R-help at r-project.org mailing list
> 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
tlumley at u.washington.edu University of Washington, Seattle
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