# zC: Sets Contrasts for a Factor

Description Usage Arguments Details Value References See Also Examples

## Description

Sets the `"contrasts"` attribute for the factor.

## Usage

 `1` ```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.

## See Also

`contrasts`, `contr.sum`, etc.

## Examples

 ``` 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15``` ```## 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) ```