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

[Charles C. Berry]

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