[R-sig-ME] Adjusting for baseline differences in growth models

Ken Beath ken at kjbeath.com.au
Fri Sep 5 00:56:31 CEST 2008

On 03/09/2008, at 1:34 PM, D Chaws wrote:

> Hi all, I posted a long question for this the other day, but no one
> has responded.  Here is a short version those of you who are very busy
> might have time to read.  Any advice would be greatly appreciated.
> I have a growth model where 2 groups differ on the outcome at
> baseline.  How do I get the growth model adjusting for this difference
> (or making the groups equivalent at baseline, such as in ANCOVA)?
> Predict growth from a 2nd(person)-level covariate of baseline scores
> (i.e., a growth:baseline covariate interaction in a mixed model)?
> Does it make sense to add the baseline covariate as a predictor of the
> intercepts as well as the slopes or is this completely redundant with
> a model where time is coded so that 0 = baseline?

Fitting a mixed-effects model for the post baseline outcomes only with  
the baseline as a covariate affecting only the intercept seems  
sensible. This is the same as the ANCOVA but with repeated measures.


> Thanks again.
> - DC
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