Hi,
I've been using gamm4 to build GAMMs for exploring environmental influences on
genetic ancestry. Things have gone well and I have 2 very straightforward
questions:
1. I've used method=REML. Am I correct that this is an alternative method for
estimating the smooth functions in GAMMs rather than GCV that is often used for
GAMs? I've read up on REML and it makes sense, but I'm confused about whether
GCV is used for any part of the model formation of a GAMM with method=REML. I
think it is not. Is this why no GCV score is returned for GAMMs through mgcv
and gamm4? For Gams I've always used the GCV scores to help with model
selection, but I assume these aren't available for GAMMs estimated with REML.
2. Instead I've been using AIC values with GAMMs. Is it correct to use the AIC
value from the underlying linear mixed model, i.e. extracted using
summary(gamm1$mer)? Or is there another AIC calculated specifically for the
gam object?
Many thanks for help with these (admittedly simple and boring) questions, I
really like the mgcv and gamm4 packages which I've found very user friendly in
conjunction with Wood (2006).
John
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