C
Sets Contrasts for a Factor
Sets the "contrasts"
attribute for the factor.
 Keywords
 models
Usage
C(object, contr, how.many, ...)
Arguments
 object
 a factor or ordered factor
 contr
 which contrasts to use. Can be a matrix with one row for
each level of the factor or a suitable function like
contr.poly
or a character string giving the name of the function  how.many
 the number of contrasts to set, by default one less
than
nlevels(object)
.  ...
 additional arguments for the function
contr
.
Details
For compatibility with S, contr
can be treatment
,
helmert
, sum
or poly
(without quotes) as shorthand
for contr.treatment
and so on.
Value

The factor
object
with the "contrasts"
attribute set.
References
Chambers, J. M. and Hastie, T. J. (1992) Statistical models. Chapter 2 of Statistical Models in S eds J. M. Chambers and T. J. Hastie, Wadsworth & Brooks/Cole.
See Also
Examples
library(stats)
## reset contrasts to defaults
options(contrasts = c("contr.treatment", "contr.poly"))
tens < with(warpbreaks, C(tension, poly, 1))
attributes(tens)
## tension SHOULD be an ordered factor, but as it is not we can use
aov(breaks ~ wool + tens + tension, data = warpbreaks)
## show the use of ... The default contrast is contr.treatment here
summary(lm(breaks ~ wool + C(tension, base = 2), data = warpbreaks))
# following on from help(esoph)
model3 < glm(cbind(ncases, ncontrols) ~ agegp + C(tobgp, , 1) +
C(alcgp, , 1), data = esoph, family = binomial())
summary(model3)
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