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.w...@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.m...@ugent.be ------------------------------- Disclaimer : http://helpdesk.ugent.be/e-maildisclaimer.php [[alternative HTML version deleted]] ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
