[R] median test

[Frank E Harrell Jr]

Please note that the median test is considered obsolete by many, because 
of its very low power.  See the reference below.  -Frank

@Article{fri00sho,
   author = 		 {Freidlin, Boris and Gastwirth, Joseph L.},
   title = 		 {Should the median test be retired from general use?},
   journal = 	 The American Statistician,
   year = 		 2000,
   volume =		 54,
   number =		 3,
   pages =		 {161-164},
   annote =		 {inefficiency of Mood median test}
}


On 05/27/2010 10:20 AM, Joshua Wiley wrote:
> Hello Linda,
>
> The "problem" is actually the median of your data.  What the function
> median.test() does first is combine both groups.  Look at this:
>
> median(c(group1, group2))
>
> the median is 1, but the lowest value of the groups is also 1.  So
> when the function does the logical check z<  m where z = c(group1,
> group2) and m is the median, there are no values that are less than
> the median value.  Therefore there is only 1 level, and the fisher
> test fails.
>
> You would either need different data or adjust the function to be:
>
> fisher.test(z<= m, g)$p.value
>
> that way it's less than or equal to the median.
>
> Hope that helps,
>
> Josh
>
> On Thu, May 27, 2010 at 7:24 AM, linda Porz<linda.porz at gmail.com>  wrote:
>> Hi all,
>>
>> I have found the following function online
>>
>> median.test<-function(y1,y2){
>>   z<-c(y1,y2)
>>   g<- rep(1:2, c(length(y1),length(y2)))
>>   m<-median(z)
>>   fisher.test(z<m,g)$p.value
>> }
>>
>> in
>>
>> http://www.mail-archive.com/r-help@r-project.org/msg95278.html
>>
>> I have the following data
>>
>>> group1<- c(2, 2, 2, 1, 4, 3, 1, 1)
>>> group2<- c(3, 1, 3, 1, 4, 1, 1, 1, 7, 1, 1, 1, 1, 1, 2)
>>> median.test(w1,group1)
>> [1] 1
>>> median.test(group1,group2)
>> Error in fisher.test(z<  m, g) : 'x' and 'y' must have at least 2 levels
>>
>> I am very thankful in advance for any suggestion and help.
>>
>> Regards,
>> Linda
>>
>>         [[alternative HTML version deleted]]
>>
>> ______________________________________________
>> R-help at 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.
>>
>
>
>


-- 
Frank E Harrell Jr   Professor and Chairman        School of Medicine
                      Department of Biostatistics   Vanderbilt University