MASS (version 7.3-56)

contr.sdif: Successive Differences Contrast Coding

Description

A coding for factors based on successive differences.

Usage

contr.sdif(n, contrasts = TRUE, sparse = FALSE)

Arguments

n

The number of levels required.

contrasts

logical: Should there be n - 1 columns orthogonal to the mean (the default) or n columns spanning the space?

sparse

logical. If true and the result would be sparse (only true for contrasts = FALSE), return a sparse matrix.

Value

If contrasts is TRUE, a matrix with n rows and n - 1 columns, and the n by n identity matrix if contrasts is FALSE.

Details

The contrast coefficients are chosen so that the coded coefficients in a one-way layout are the differences between the means of the second and first levels, the third and second levels, and so on. This makes most sense for ordered factors, but does not assume that the levels are equally spaced.

References

Venables, W. N. and Ripley, B. D. (2002) Modern Applied Statistics with S. Fourth Edition, Springer.

See Also

contr.treatment, contr.sum, contr.helmert.

Examples

# NOT RUN {
(A <- contr.sdif(6))
zapsmall(ginv(A))
# }