[R] Centered variables and mixed-model

[Douglas Bates]

Martin Julien wrote:
> 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?

Well I don't recall Pinheiro and Bates saying that variables needed to 
be centered on their median.  It is often the case that the conditioning 
of the numerical optimization for obtaining parameter estimates is 
improved if explanatory variables are centered in some way but I don't 
know of a particular reason for centering on the median.  Also, as Bert 
Gunter pointed out, this statement is about the explanatory variables 
and not a response variable.

With any statistical model, centering of variables results in a 
reparameterization of the model and one must keep this in mind when 
performing statistical tests.