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.