Re: [R] Orthogonal polynomial contrasts

From: Frank E Harrell Jr <f.harrell_at_vanderbilt.edu>
Date: Thu, 10 Apr 2008 08:21:49 -0500

Peter Dalgaard wrote:
> Chris Bennett wrote:

>> How do you remove one of the terms from an ordered polynomial contrast in
>> your linear model.  For example, I have significant terms for linear and
>> cubic but not quadratic, how would i remove the quadratic term from
>> lm(response~treatment)
>>
>> Cheers,
>> Chris
>>   

> Are you _sure_ you want to do that? The interpretation can get tricky,
> but if you insist, just set the contrast matrix for the term
> accordingly, e.g.,
>
> C(myterm, contr.poly(4)[,-2])
>

Related to Peter's note of caution, using statistical significance as a basis for this decision is asking for trouble. See

   author = {Grambsch, P. M. and {O'Brien}, P. C.},    year = 1991,
   title = {The effects of transformations and preliminary tests for

           non-linearity in regression},    journal = Stat in Med,
   volume = 10,
   pages = {697-709}

Frank

-- 
Frank E Harrell Jr   Professor and Chair           School of Medicine
                      Department of Biostatistics   Vanderbilt University

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Received on Thu 10 Apr 2008 - 13:24:38 GMT

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