[R] How to compute contrast where there are interaction terms in the linear model?

[Peng Yu]

On Tue, Dec 1, 2009 at 4:04 PM, Charles C. Berry <cberry at tajo.ucsd.edu> wrote:
> On Tue, 1 Dec 2009, Peng Yu wrote:
>
>> Could somebody recommend some textbook how to compute contrast when
>> there are interactions terms? "Applied Linear Regression Models"
>> (book) mentioned contrast, but I cannot extend it to the case where
>> there are interaction terms.
>
>
> Textbook? Schmextbook!
>
> You have the power of R at your fingertips!
>
> Use it to explore concepts you are trying to wrap your brain around!
>
>
>> df <- expand.grid(x1=1:20,x2=factor(letters[1:2]))
>> vanilla <- model.matrix(~0+poly(x1,degree=3), df )
>> matplot(row( vanilla) , vanilla, type='b')
>> inter <-  model.matrix(~0+poly(x1,degree=3):x2, df )
>> matplot(row( inter ) , inter, type='b')

I don't understand. Where the contrast is?

BTW, what does 'schmextbook' mean?