contrasts                package: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)

