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.
- The factor
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)