| 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 |
value |
either a numeric matrix whose columns give coefficients for
contrasts in the levels of |
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.
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
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)