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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