contrasts {base}

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) <- ctr

Arguments

x	a factor.
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.
ctr	either a 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.

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.

Note

Prior to R version 1.2.0, contrasts(, FALSE) called a contrasts function with contrasts = FALSE. This normally gave the same result, but not for contr.poly, the default for ordered factors.

See Also

C, contr.helmert, contr.poly, contr.sum, contr.treatment; glm, aov, lm.

Examples

example(factor)
(fff <- factor(ff))
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)

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