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:cbe...@tajo.ucsd.edu UC San Diego
http://famprevmed.ucsd.edu/faculty/cberry/ La Jolla, San Diego 92093-0901
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