Hi all,
Thank you for your reply.
if done properly! What does this mean? The R-code I have is using the
R-function sample without replacement. Am I doing this properly?
median of the differences is zero! Does this mean if I run 1000 permutation
and for each permutation I compute the median difference and as a result I
have 1000 differences. Is the the H0: median(1000 differences) =0? If yes,
which conclusion one would have from this H0?
Best wishes,
Cheba
2010/5/7 Joris Meys
> 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 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
>>>
>>>
>>>>
>>>> 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>>> }
>>>>
>>>> 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
>>>> tlumley@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
>> tlumley@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.
>>
>
>
>
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> Joris Meys
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>
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