[R] Where is gam?
Jari Oksanen
jhoksane at ecology.helsinki.fi
Wed Oct 4 07:22:44 CEST 2000
aprasad at fs.fed.us said:
> Could you please clarify.... there has been a lot of interest in gam
> recently in predicting species distribution and environmental
> management (there is in fact a workshop scheduled in Switzerland next
> year on GLM/GAM modelling).
I think that if there is one field where GAMs are "oversold" that must be
(vegetation) ecology and modelling species responses along environmental
variables (that we call -- believe or not -- "gradients"). GAMs are used since
some people are worried about the "correct shape" of species response
functions, and in particular, against the symmetric Gaussian response
functions. In particular they are worried about things like skewness of the
species responses. GAMs are used since they seem to impose no constraint in
the shape and so you can see if your responses *really* are symmetric of if
they are skewed, flat or in other interesting shapes. However, these
questions about shape are parametric questions and they cannot be answered
with non-parametric models, but these must be subjectively interpreted in
parametric terms. It seems that vegetation ecologists are more GAM-fixed that
they do not even follow the example of Hastie & Tibshirani book where GAMs
were regularly compared against other models, and quite often, used to select
a credible parametric model. The major problem in vegetation ecology is that
first we use GAM, get a response curve, and then we interpret these shapes in
parametric terms. So we say, e.g., that we found a skewed response without
comparing this shape to a symmetric model. Question about skewness is a
parametric question (`skewness' *is* a parameter), and so we need nested
models which differ with this models. We cannot use non-parametric models and
give them parametric interpretation (`non-parametric' in the sense that the
parameters there are nothing we could use for interpretation).
In particular in exploratory vegetation ecology where we go out and observe
and measure things, we can hardly ignore interactions either.
At the moment we are somewhat short of models which could be used for the
parametric questions of veg.ecologists. I have made some trials with these:
@article{Huisman93,
author = {J. Huisman and H. Olff and L. F. M. Fresco},
title = {A hierarchical set of models for species response analysis},
journal = {Journal of Vegetation Science},
volume = {4},
pages = {37-46},
year = {1993}
}
It should be easy to fit their models in R, even with ML, although I have done
this so far only with a program written specifically for these models (because
I did not know about R when I started).
cheers, Jari Oksanen
--
"Infinite variety of Nature, that is pure myth. It is not found in
Nature herself. It resides in the imagination, or fancy, or cultivated
blindness of the man who looks at her." Oscar Wilde
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