Re: [R] question for aov and kruskal

From: David Hewitt <dhewitt37_at_gmail.com>
Date: Tue, 11 Mar 2008 07:31:01 -0700 (PDT)

> 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?
>

You shouldn't have needed aov to tell you that the data (not the model) are unbalanced. I could see that without running the code! Seriously, you might need to think more about the type of model you're using, and what you want to know, and then consider how to estimate the effect sizes of interest.



David Hewitt
Virginia Institute of Marine Science
http://www.vims.edu/fish/students/dhewitt/
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