C {stats}  R Documentation 
Sets the "contrasts"
attribute for the factor.
C(object, contr, how.many, ...)
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

how.many 
the number of contrasts to set, by default one less
than 
... 
additional arguments for the function 
For compatibility with S, contr
can be treatment
,
helmert
, sum
or poly
(without quotes) as shorthand
for contr.treatment
and so on.
The factor object
with the "contrasts"
attribute set.
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.
## 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)