# [R] median of two groups

Thomas Lumley 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.

-thomas

>
> Regards,
> Cheba
>
> 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.
>>
>> 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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>>>
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>>>
>> Thomas Lumley                   Assoc. Professor, Biostatistics
>> tlumley at u.washington.edu        University of Washington, Seattle
>>
>>
>
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>
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