R help - Jul 2007 - Subsetting problem

I need to perform the Exact Wilcoxon Mann-Whitney on a subset of my database.
Assuming that IPPO is my data frame and IPPOBIS is the subset my variable still
have 3 different levels and the function wilcox_test (package "coin")
does not accept it. 
I do not know how to overcome this problem.

ippo <- c(rep("A",10),rep("B",10),rep("C",10))
ippo2 <- c(rnorm(10,0,1),rnorm(10,10,10),rnorm(10,10,10))
IPPO <- data.frame(ippo,ippo2)

IPPOBIS <- IPPO[IPPO$ippo == "A" | IPPO$ippo == "B",]

wilcox_test(ippo2 ~ ippo,data=IPPOBIS,distribution=exact())
Error in check(itp) : 'object' does not represent a two sample problem
levels(IPPOBIS$ippo)
[1] "A" "B" "C"

Massimo Cressoni

You have three levels of factor 'ippo' and data on two.  That is not a 
two-sample problem, as the error message says. Try

IPPOBIS$ippo <- IPPOBIS$ippo[drop=TRUE]

And please use an informative subject line (see the posting guide).

On Thu, 12 Jul 2007, Cressoni, Massimo (NIH/NHLBI) [F] wrote:
> I need to perform the Exact Wilcoxon Mann-Whitney on a subset of my
database.
> Assuming that IPPO is my data frame and IPPOBIS is the subset my variable
still
> have 3 different levels and the function wilcox_test (package
"coin")
> does not accept it.
> I do not know how to overcome this problem.
>
> ippo <-
c(rep("A",10),rep("B",10),rep("C",10))
> ippo2 <- c(rnorm(10,0,1),rnorm(10,10,10),rnorm(10,10,10))
> IPPO <- data.frame(ippo,ippo2)
>
> IPPOBIS <- IPPO[IPPO$ippo == "A" | IPPO$ippo ==
"B",]
>
> wilcox_test(ippo2 ~ ippo,data=IPPOBIS,distribution=exact())
> Error in check(itp) : 'object' does not represent a two sample
problem
> levels(IPPOBIS$ippo)
> [1] "A" "B" "C"
>
> Massimo Cressoni
>
> ______________________________________________
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http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>
-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
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