[R] gam questions

[Simon Wood]

> 2. I've fitted a glm y~a+x+a:x, where a is a 3 level factor and x is a 
> continuous covariate. If I want to fit a similar gam model, is it correct 
> to fit y~a+s(x)+s(x,by=a.1)+s(x,by=a.2)+s(x,by=a.3), where a.1--a.3 are 
> dummy variables representing each level of the factor? Or is the s(x) term 
> redundant?
- yes the s(x) term is redundant and in the current mgcv version will
likely cause spectacular nonsense as a result of lack of identifiability
in the smooth part of the model (mgcv 0.9 will cope with this when
released, but it's still much better to use an identifiable smooth
model). 

Simon
_____________________________________________________________________
> Simon Wood simon at stats.gla.ac.uk        www.stats.gla.ac.uk/~simon/
>>  Department of Statistics, University of Glasgow, Glasgow, G12 8QQ
>>>   Direct telephone: (0)141 330 4530          Fax: (0)141 330 4814