[R] Theta in Negative binomial GAM

[Eva Maria Leunissen]

Hi, I'm fitting a negative binomial GAM (using mgcv) to my data using
family=nb() so theta is estimated during the fitting process. When I then
extract this theta from the model and refit the same model with
family=negbin(theta) it gives a much lower AIC. I know using AIC to compare
negative binomial models should be done with caution (
http://r.789695.n4.nabble.com/How-to-compare-GLM-and-GAM-models-tt827923.html#a827926)
but approximately it is ok. My questions are:

Is it better to do model selection with nb() or with negbin and a known
theta. if the latter how do you know what theta is?

can you compare models using AIC if the theta is different for each model?

Thanks in advance, any help is much appreciated

Kind regards,

Eva Leunissen

	[[alternative HTML version deleted]]