[R] how to add in interaction terms in gamm

[Simon Wood]

The usual method for producing interactions of smooths with factors in 
mgcv is using `by' variables (see ?gam.models for some documentation). 
For example
if `gender' is a two level factor then

... s(time,by=gender) + gender

would produce a smooth of time for each gender. If you want a different 
smooth of time for each combination of gender and gene, then I'd 
recommend producing a factor with a level for each gender gene 
combination, and using that as the by variable.

Simon

On 23/03/11 18:34, straybird_tx wrote:
> I want to use gamm to generate smoothed trend line for three groups
> identified by dummy variable genea and geneb. My question is how to add in
> an interaction term between the time and another dummy variable such as
> gender?
> fitm<-gamm(change_gfr~
> genea+geneb+s(timea_n,bs="ps")+s(timeb_n,bs="ps")+s(timec_n,bs="ps"),data=mm,random=list(time_n=~1|PID))
>
>
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-- 
---
Simon Wood, Mathematical Science, University of Bath BA2 7AY UK
+44 (0)1225 386603               http://people.bath.ac.uk/sw283