[R] interactions in GAMs

[Simon Wood]

David,

I think that the colorsex approach is the right one, and colorsex should 
initially be included as a main effect, because the smooths are centred 
for factor by variables (see e.g. ?gam.models). Whether you then choose 
to drop this main effect, as it appears to be non-significant, is a 
matter of taste (I would tend to leave it in).

best,
Simon

On 27/11/12 16:42, chirleu wrote:
> Hi all,
> I wonder if it's possible to include a double interaction in a GAM formula.
> Example:
>
> If I do this:
> mod=gam(energy~s(size, *by=color, by=sex*, k=5) + temperature, ...)
>
> I get the interaction betwen size*color and size*sex.
>
> But I need size*color*sex, being size a smoother.
> I've created a new variable (colorsex) which combines all the level of both
> color (2 levels) and sex (2 level), so that I have a new variable with 4
> level. In this case I can do:
>
> mod=gam(energy~s(size, *by=colorsex*, k=5) + temperature, ...)
>
> What do you think of this approach?
>
> In this case, should I also include colorsex (or color*sex) in the
> parametric term *even if it's not significant*(as it's the case)?
>
> Many thanks
>
> David
>
>
>
>
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>
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>
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-- 
Simon Wood, Mathematical Science, University of Bath BA2 7AY UK
+44 (0)1225 386603               http://people.bath.ac.uk/sw283