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)$p.value
that way it's less than or equal to the median.
Hope that helps,
Josh
On Thu, May 27, 2010 at 7:24 AM, linda Porz<linda.p...@gmail.com> wrote:
Hi all,
I have found the following function online
median.test<-function(y1,y2){
z<-c(y1,y2)
g<- rep(1:2, c(length(y1),length(y2)))
m<-median(z)
fisher.test(z<m,g)$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
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