Thank you for the correction. I was thinking about the difference of using a
variable with a smoother, and comparing that to a model with that variable
without smoother. I should specify that I mostly use thin plate regression
splines.

If the spline itself is not deviating from linearity, you get a nice
straight line going through the point (mean(Data),0) if you look at the
marginal plots. Use the same variable unchanged but as a simple linear
effect in the model, and the same line will run through (0,0). At least,
that's what I noticed and also the reason why I center my variables first.
The models are essentially the same, the shift is mainly in the intercept.
But the centering got a bit a reflex.

Cheers
Joris

On Wed, May 19, 2010 at 8:20 PM, Simon Wood <s.wood@bath.ac.uk> wrote:

> On Wednesday 19 May 2010 15:29, Joris Meys wrote:
> > Could you specify the package you use? If it is mgcv, this one centers
> your
> > variables before applying the smooths. That's something to take into
> > account when comparing different models.
> --- er, actually it only centres variables in this way for some smoothing
> bases, for numerical stability purposes: but this is done in a way that is
> user transparent and makes absolutely no difference to model interpretation
> or comparison. Of course the smooths themselves are subject to `centering
> constraints'  (but that's very different to centering the variables) ---
> these are just identifiability constraints --- all gam fitting packages
> have
> to put some identifiability constraints on the smooths, and the centering
> constraints used by `mgcv' and `gam' have the benefit of minimizing the
> standard errors on the constrained smooths.
>
> best,
> Simon
>
> --
> > Simon Wood, Mathematical Sciences, University of Bath, Bath, BA2 7AY UK
> > +44 1225 386603  www.maths.bath.ac.uk/~sw283<http://www.maths.bath.ac.uk/%7Esw283>
>



-- 
Joris Meys
Statistical Consultant

Ghent University
Faculty of Bioscience Engineering
Department of Applied mathematics, biometrics and process control

Coupure Links 653
B-9000 Gent

tel : +32 9 264 59 87
Joris.Meys@Ugent.be
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