Sets Contrasts for a Factor

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).
... Addtional arguments for the function contr.

Description

Sets the "contrasts" attribute for the factor.

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 with the "contrasts" attribute set.

Author(s)

B.D. Ripley

See Also

contrasts

Examples

## reset contrasts to defaults
options(contrasts=c("contr.treatment", "contr.poly"))
data(warpbreaks)
attach(warpbreaks)
tens <- C(tension, poly, 1)
attributes(tens)
detach()
## 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))

data(esoph) # 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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