[R] caret: compare linear models of different degree

[Dominik Bruhn]

I want to use the caret package to train linear models. I want to
compare these models when using different degrees (aka degrees of
interaction). This is possible for the 'earth' method (using the
'.degree' parameter) but I found no possibility of customizing the
degree for the 'lm' method. This might be due to the fact that the basic
'lm' function does not support the degree parameter.

I tried the following solution using a different formula for the second
training:
-----
library(caret)
data(trees)
formula=Volume~Girth+Height
degFormula=Volume~(Girth+Height)^2
m1 = train(formula, data=trees, method="lm")
m2 = train(degFormula, data=trees, method="lm")
-------

The problem with this solution is that the 'extractPrediction' method of
R does not work if comparing models with different formulas:

---
Error in eval(expr, envir, enclos) : object 'Girth:Height' not found
---

How can I set the degree used when training a linear model without
changing the formula? I need the 'extractPrediction' functionallity.

Thanks!
-- 
Dominik Bruhn
mailto: dominik at dbruhn.de