| contrasts {stats} | R Documentation |
Set and view the contrasts associated with a factor.
contrasts(x, contrasts = TRUE) contrasts(x, how.many) <- value
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 ctr. |
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. |
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.
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.
C,
contr.helmert,
contr.poly,
contr.sum,
contr.treatment;
glm,
aov,
lm.
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)