[R] Centered variables and mixed-model

[Martin Julien]

I work in biology and I use mixed-model for my data analysis 
In a scientific paper, the author wrote: 
"All continuous exploratory variables were centred on their median value
prior to inclusion in the analysis (Pinheiro & Bates, 2000)."
They refer to the book "Mixed-effects model in S and S-Plus" by Pinheiro et
Bates in 2000.
I feel a bit strange with that paper because I can't find in the book why
they centred the variables on their median.
So I have two question:
First, is it correct to centred the variables on their median in a
mixed-model?
Second, why they do that?
Thank
Julien 

-----Message d'origine-----
De : Liaw, Andy [mailto:andy_liaw at merck.com] 
Envoyé : 13 janvier 2005 19:26
À : 'Martin Julien'; r-help at stat.math.ethz.ch
Objet : RE: [R] Lme and centered variables

I'm by no means expert in such matter, but where did you get the idea that
such manipulation is needed?  If you center the response by its mean, that's
the same as taking out the intercept.  There's no reason to do that
beforehand.  As to centering by median, that's a first for me, and quite
frankly I don't see what it buys you.

Andy

> From: Martin Julien
> 
> I want to fit a linear mixed-model with my data but I want to 
> know if I have
> to center all my responses variables on their median and why 
> I have to do
> it?
> 
> Thank
> 
> Julien Martin
> 
>  
> 
> 
> 	[[alternative HTML version deleted]]
> 
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