[R] mood.test/mood.medtest

[peter dalgaard]

> On 15 Mar 2017, at 16:32 , Leemann, Lucas T <l.leemann at essex.ac.uk> wrote:
> 
> Hello,
> 
> I was trying to test whether two medians are identical or not and used the function “mood.test” from the “stats" package. My co-author, a medical doctor, was trying to do the same in SPSS and had different results.

stats::mood.test() is a test of scale, not medians, according to its documentation. 

mood.medtest() is a test for a common median, basically looking at a crosstable of observations above and below the joint median:

> M <- table(indicator, x > median(x))
> chisq.test(M)

	Pearson's Chi-squared test with Yates' continuity correction

data:  M
X-squared = 4.125, df = 1, p-value = 0.04225
> mood.medtest(x ~ indicator)

	Mood's median test

data:  x by indicator
X-squared = 4.125, df = 1, p-value = 0.04225

This might differ from SPSS output (which you do not cite) in details like Yates correction, use of exact test, etc.

-pd



> 
> I wanted to see whether there was a problem on my end and also used the function “mood.medtest” from the “RVAideMemoire” package. I find different results while I am under the impression that both functions claim to carry out the same test and have the same defaults. While my actual data is sensitive medical information, I provide simple code below for a reproducible example. 
> 
> library(RVAideMemoire)
> set.seed(123)
> a <- runif(100)
> b <- runif(120,0.2,1.1)
> indicator <- c(rep(0,100),rep(1,120))
> x <- c(a,b)
> mood.test(x ~ indicator)
> mood.medtest(x ~ indicator)
> 
> Has anybody encounter this problem before or would be able to provide any insights?
> 
> Best wishes,
> Lucas
> 
> 
> 
> 
> 
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-- 
Peter Dalgaard, Professor,
Center for Statistics, Copenhagen Business School
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Email: pd.mes at cbs.dk  Priv: PDalgd at gmail.com