R help - Dec 2009 - How to compute contrast where there are interaction terms in the linear model?

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.

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')
>

Chuck


Charles C. Berry                            (858) 534-2098
                                             Dept of Family/Preventive Medicine
E mailto:cberry at tajo.ucsd.edu	            UC San Diego
http://famprevmed.ucsd.edu/faculty/cberry/  La Jolla, San Diego 92093-0901