contrasts {stats} | R Documentation |

## Get and Set Contrast Matrices

### Description

Set and view the contrasts associated with a factor.

### Usage

contrasts(x, contrasts = TRUE)
contrasts(x, how.many) <- value

### Arguments

`x` |
a factor or a logical variable. |

`contrasts` |
logical. See Details. |

`how.many` |
How many contrasts should be made. Defaults to one
less than the number of levels of `x` . This need not be the
same as the number of columns of `ctr` . |

`value` |
either a numeric matrix whose columns give coefficients for
contrasts in the levels of `x` , or the (quoted) name of a
function which computes such matrices. |

### Details

If contrasts are not set for a factor the default functions from
`options("contrasts")`

are used.

A logical vector `x`

is converted into a two-level factor with
levels `c(FALSE, TRUE)`

(regardless of which levels occur in the
variable).

The argument `contrasts`

is ignored if `x`

has a matrix
`contrasts`

attribute set. Otherwise if `contrasts = TRUE`

it is passed to a contrasts function such as
`contr.treatment`

and if `contrasts = FALSE`

an identity matrix is returned.

### 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

`C`

,
`contr.helmert`

,
`contr.poly`

,
`contr.sum`

,
`contr.treatment`

;
`glm`

,
`aov`

,
`lm`

.

### Examples

example(factor)
fff <- ff[, drop=TRUE] # reduce to 5 levels.
contrasts(fff) # treatment contrasts by default
contrasts(C(fff, sum))
contrasts(fff, contrasts = FALSE) # the 5x5 identity matrix
contrasts(fff) <- contr.sum(5); contrasts(fff) # set sum contrasts
contrasts(fff, 2) <- contr.sum(5); contrasts(fff) # set 2 contrasts
# supply 2 contrasts, compute 2 more to make full set of 4.
contrasts(fff) <- contr.sum(5)[,1:2]; contrasts(fff)

[Package

*stats* version 2.5.0

Index]