R: Sets Contrasts for a Factor

C {stats} | R Documentation

## Sets Contrasts for a Factor

### Description

Sets the `"contrasts"` attribute for the 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)`.
`...` | 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

    ## 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)

[Package stats version 3.1.2 Index]