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

[Charles C. Berry]

On Fri, 4 Dec 2009, Peng Yu wrote:

> 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?

The numbered curves are the terms of polynomial contrasts.

See

 	?poly
 	?contr.poly
and note

 	all( contr.poly(5) == poly(1:5,degree=4) )




>
> BTW, what does 'schmextbook' mean?
>

Well, repeating a word and prepending 'schm' (in place of the initial 
consonant sound, if any) is a slang idiom for asserting low regard for the 
idea expressed by the word.

As in 'Birthday-Schmirthday' when moaning about getting older:

 	http://www.berkeleydailyplanet.com/issue/2009-01-29/article/32134?headline=First-Person-Birthday-Schmirthday-Whinings-on-Mortality


or  'Middle-schmiddle' which expresses low regard for the 'middle' 
policy, see

 	http://aussiethule.blogspot.com/2006/03/olmerts-middle-schmiddle.html

Or 'Nilsson-Schmilsson', a self-deprecating title for an album by Harry 
Nilsson:

 	http://en.wikipedia.org/wiki/Nilsson_Schmilsson

Or 'textbook-schmextbook', in which I express the idea that a textbook 
really isn't needed.

Chuck

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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