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 value.
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.

If value supplies more than how.many contrasts, the first how.many are used. If too few are supplied, a suitable contrast matrix is created by extending value after ensuring its columns are contrasts (orthogonal to the constant term) and not collinear.

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

utils::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)

---