[R] Coding of categorical variables for logistic regression?

[Daniel Malter]

This is about the covariates, not the dependent variable. So a polynomial logistic regression seems hardly appropriate. 

David is right with his latest statement, just because they are ordered does not assure that the effect is monotonic. If the "low", "medium", and "high" groups had even spacing (e.g., 10, 20, 30 (on whatever scale)), you could use an alternative way of factor coding. Orthogonal polynomial contrasts tests for curvilinear effects by while keeping the factor nature of the variable intact. That is, it is not the same as including the numeric variable and its higher order polynomials (e.g., Rosenthal and Rosnow 1985).

Daniel

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cuncta stricte discussurus
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-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf Of C.H.
Sent: Sunday, March 28, 2010 11:10 PM
To: Ravi Kulkarni
Cc: r-help at r-project.org
Subject: Re: [R] Coding of categorical variables for logistic regression?

?polr of the MASS package.


On Mon, Mar 29, 2010 at 10:39 AM, Ravi Kulkarni <ravi.kulk at gmail.com> wrote:
>
> How do I use the "extra" information that two of my predictors are ordinal?
> (I did not know I could do that.)
>
> Thanks,
>  Ravi
> --
> View this message in context: http://n4.nabble.com/Coding-of-categorical-variables-for-logistic-regression-tp1694250p1694644.html
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-- 
CH Chan
Research Assistant - KWH
http://www.macgrass.com

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