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st: RE: Covariates in ANOVAs


From   "Joseph Coveney" <jcoveney@bigplanet.com>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: RE: Covariates in ANOVAs
Date   Fri, 15 May 2009 12:48:04 +0900

Miss Gina Micke wrote:

I would like to know how to control for an independent continuous variable when
using a repeated measures ANOVA. I have a 2x2 factorial design experiment with
males and females in each of the treatment groups (T1 & T2) measured at multiple
"events", each subject assigned an "id". The subjects are all different ages so
I would like to be able to control for the effect of "age range".

If I didn't have "age range" as a covariate my model would be as follows:

anova x T1 T2 sex T1*T2 T1*sex T2*sex T1*T2*sex/id|T1*event T2*event T1*T2*event
sex*event sex*T1*event sex*T2*event T1*T2*sex*event, repeated(event) partial
bse(id|T1*T2*sex)


Do I use the ANCOVA and cont(age range) command? Is this output the effects of
my fixed factors taking into account the effect of age range?

--------------------------------------------------------------------------------

First, the command line text shown doesn't look at all like correct syntax.
Second, by the name of it, a variable called "age range" would be categorical
and so you would not be using the -continuous()- option for it in -anova-.

Unless you have balanced data (equal representation of age ranges within each
factorial cell, as well as equal sex-by-treatment group cell sizes maintained
throughout the observation period), try something like the following to start
with:

xi3 i.treatment_group*i.age_group*i.sex*i.observation_interval
xtmixed response _I* || patient_id: , nolrtest variance reml

You'll need to install the user-written command, -xi3-, from SSC if you haven't
already.

Joseph Coveney



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