R help - Nov 2011 - ANOVA on residuals to check for equality of variances

Hi all,

I know that I can run an one way ANOVA on the absolute residual values
to check the assumption of equal variances:
model<- lm(y~x)
summary(lm(abs(model$resid~x)))

What if I have two factors? As far as I know I have to check this
assumption on very factor/level combination. So if I have 3 factors with
three levels, I would have to test 9 factor/level combinations. Here
comes my questions:

Can I do the same thing like above, except, including the "multiply"
sign between the factors, check the overall significance (for equal
variances) and go from there?

model<- lm(y~x1*x2)
summary(lm(abs(model$resid~x1*x2))).

Right now I am using levene.test() from the 'lawstat' package and just
adding another column to my data:
mydata$group<-rep(letters[1:9],each=5), where letters[1:9] are for 9
factor/level combination and each=5 since I have 5 replicates each. Then
I run levene.test(mydata$y, mydata$group, location="mean") and it
works
fine.

Thanks for any answers on that!

Stefan



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