## 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 2.5.0

Index]