[R] Qualitative factors

John Fox jfox at mcmaster.ca
Thu Nov 7 15:10:18 CET 2002

Dear Isabelle,

Contrast coding in R for factors and ordered factors is governed by the 
contrasts option:

     > options("contrasts")
             unordered           ordered
     "contr.treatment"      "contr.poly"

The default for an unordered factor is "treatment" (0/1) coding, using the 
first level as the baseline:

     > contrasts(m)
       2 3
     1 0 0
     2 1 0
     3 0 1

     > options("contrasts")
             unordered           ordered
     "contr.treatment"      "contr.poly"

See ?contr.treatment for details and alternatives.

I hope that this helps,

At 09:55 AM 11/7/2002 +0100, you wrote:
>I have some doubt about how qualitative factors are coded in R. For
>instance, I consider a response y, a quantitative factor x and a qualitative
>factor m at 3 levels, generated as follow :
>(Intercept)            x           m2           m3
>     3.96364      0.09818      0.44145      0.62291
>In literature, 2 usual implicit coding process are suggested : m1=0 or
>m1=-m2-m3. Does R use one of these process ? (I've already read the R
>documentation on this topic, and it is still not clear in my mind).
>Furthermore, how can I make prediction using this model, I mean how should I
>specify the new data (especially for m).

John Fox
Department of Sociology
McMaster University
Hamilton, Ontario, Canada L8S 4M4
email: jfox at mcmaster.ca
phone: 905-525-9140x23604
web: www.socsci.mcmaster.ca/jfox

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