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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