R help - Jun 2012 - caret: compare linear models of different degree

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

Max, thanks for your answer!
> predict.train() will handle the formulas. If you want to compare the
> models in terms of their predictive performance, set the seeds prior
> to running the model. This will ensure that the same resampling
> indices are used in train(). If you do this, the resamples() function
> can be used to make formal comparisons between the models:
I think I did not express my question in the right way: I want to use
the extractPrediction function because I want to plot some stats using
the plotObsVsPred method. As stated in my previous mail, the
extractPrediction method does not work if the formula is changed.

Perhaps my question now got clearer.

Thanks again!

-- 
Dominik Bruhn
mailto: dominik at dbruhn.de