# C

##### Sets Contrasts for a Factor

Sets the `"contrasts"`

attribute for the factor.

- Keywords
- models

##### 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

##### Examples

`library(stats)`

```
## 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)
```

*Documentation reproduced from package stats, version 3.3, License: Part of R 3.3*

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