[R] question for aov and kruskal

[David Winsemius]

Subject: Re: question for aov and kruskal
Newsgroups: R-help:gmane.comp.lang.r.general
To: eugen pircalabelu <eugen_pircalabelu at yahoo.com>

On 10 Mar 2008, you wrote in gmane.comp.lang.r.general:

> Hi R users!
> 
> I have the following problem: how appropriate is my aov model under
> the violation of anova assumptions? 
> 
> Example:
> a<-c(1,1,1,1,1,1,1,1,1,1,2,2,2,3,3,3,3,3,3,3)
> b<-c(101,1010,200,300,400, 202, 121, 234, 55,555,66,76,88,34,239,
> 30, 40, 50,50,60) z<-data.frame(a, b)
> fligner.test(z$b, factor(z$a))
> aov(z$b~factor(z$a))->ll
> TukeyHSD(ll)
> 
> Now from the aov i found that my  model is unbalanced, and from the
> flinger test  i found out that the assumption of homogeneity  of
> variances is rejected. Could my Tukey comparison be a valid one
> under these violations? From what i read the Tukey test is valid
> only when the model is balanced and when the assumption of
> homogeneity of variances is not rejected, am i wrong? Can anyone
> tell me what would be the correct test in this case?  
> 
> Doing a non-parametric Kruskal - wallis test would give me a
> different result. But what would be the correct multiple comparison
> test in this case? 
> 

If you install the coin package and look at ?oneway_test help panel, 
you 
will see an implementation of what the authors (citing Hollander and 
Wolfe) are calling the Nemenyi-Damico-Wolfe-Dunn test. From the example 
it appears that you also need the multcomp package to run the test.

-- 
David Winsemius