depends on how you interprete "absolute median difference". Is that the
absolute difference of the medians, or the median of the absolute
differences. Probably the latter one, so you would be right. If it's the
former one, then it is testing whether the difference of the medians is
zero.
Cheers
Joris
On Fri, May 7, 2010 at 6:52 PM, Thomas Lumley <tlum...@u.washington.edu>wrote:
> 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
>>>>
>>>> [[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
>> 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
>
> ______________________________________________
> 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.
>
--
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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