[R-sig-ME] Mixed-models and condition number

[Christina Bogner]

Dear list members,

I'm working with both nlme and lme4 packages trying to fit linear 
mixed-models to soil chemical and physical data. I know that for linear 
models one can calculate the condition number kappa of the model matrix 
to know whether the problem is well- or ill-conditioned. Does it make 
any sense to compute kappa on the design matrix of the fixed-effects in 
nlme or lme4? For comparison I fitted a simple linear model to my data 
and scaling some numerical predictors decreased kappa considerably. So I 
wonder if scaling them in the mixed-model has any advantages?

Thanks a lot for your help.

Christina Bogner

-- 
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Christina Bogner
D8

Ecological Modelling
Soil Physics Group
University of Bayreuth
Dr.-Hans-Frisch-Straße 1-3
D-95440 Bayreuth
Germany

Tel:	++49 (0)921 555655
Fax:	++49 (0)921 555799
Mail:	christina.bogner at uni-bayreuth.de
Web:	http://www.bitoek.uni-bayreuth.de/mod/