[R] Mixed ANCOVA with between-Ss covariate?

[Mike Lawrence]

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. Did my request run afoul of the list etiquette?

On Tue, Feb 10, 2009 at 5:12 PM, Mike Lawrence <mike at thatmike.com> 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. ~
>



-- 
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. ~