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