# C

0th

Percentile

##### 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.

contrasts, contr.sum, etc.
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)