[R] Mixed ANCOVA with between-Ss covariate?

[Mark Difford]

Hi Mike,

>> I'm surprised there were no takers on this query; I thought it would 
>> be an easy answer, particularly where I provided example data set and 
>> code.

The following simplification should help you to answer your own question.

covariate_aov = aov(dv~(covariate+group+iv1+iv2)^2,data=a)

You can find out more about how to specify models in Chap. 11 of the manual
"An Introduction to R."

>> Did my request run afoul of the list etiquette?

I don't believe so. You can never be sure that someone who knew the answer
saw your first posting.

Regards, Mark.


Mike Lawrence-7 wrote:
> 
> Hi all,
> 
> I have data from an experiment with 3 independent variables, 2 are
> within and 1 is between. In addition to the dependent variable, I have
> a covariate that is a single measure per subject. Below I provide an
> example generated data set and my approach to implementing the ANCOVA.
> However the output confuses me; why does the covariate only appear in
> the first strata? Presumably it should appear in every strata in which
> there can be between-Ss effects or interactions with between-Ss
> effects, no?
> 
> #generate data
> set.seed(1)
> a=rbind(
> 	expand.grid(
> 		id=1:20
> 		,iv1 = 1:2
> 		,iv2 = 1:2
> 	)
> )
> a$group = factor(a$id<11)
> a$dv = rnorm(length(a[,1]))
> a$covariate = NA
> for(i in unique(a$id)){
> 	a$covariate[a$id==i]= rnorm(1)
> }
> 
> #make sure id, iv1, and iv2 are factorized
> a$id=factor(a$id)
> a$iv1=factor(a$iv1)
> a$iv2=factor(a$iv2)
> 
> #run ANCOVA
> covariate_aov = aov(dv~covariate+group*iv1*iv2+Error(id/(iv1*iv2)),data=a)
> summary(covariate_aov)
> 
> 
> -- 
> Mike Lawrence
> Graduate Student
> Department of Psychology
> Dalhousie University
> www.thatmike.com
> 
> Looking to arrange a meeting? Check my public calendar:
> http://www.thatmike.com/mikes-public-calendar
> 
> ~ Certainty is folly... I think. ~
> 
> ______________________________________________
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> PLEASE do read the posting guide
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> and provide commented, minimal, self-contained, reproducible code.
> 
> 

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