I am using GAM to model time trends in a logistic regression. Yet I would like to extract the the fitted spline from it to add it to another model, that cannot be fitted in GAM or GAMM.

Thus I have 2 questions:

  1. How can I fit a smoother over time so that I force one knot to be at a particular location while letting the model to find the other knots?

  2. How can I extract the matrix from the fitted GAM so that I can use it in as an impute for a different model?

The types of models I am running are to the following form:

gam <- gam(mortality.under.2~ maternal_age_c+ I(maternal_age_c^2)+
           s(birth_year,by=wealth2) + wealth2 + sex +
           residence + maternal_educ + birth_order,
           data=colombia2, family="binomial")

I've read the extensive documentation for the GAM but I am not sure still. Any suggestion is really appreciated.

r gam mgcv
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