| contrast {base} | R Documentation |
Contrast Matrices
Description
Return a matrix of contrasts.
Usage
contr.helmert(n, contrasts = TRUE)
contr.poly(n, contrasts = TRUE)
contr.sum(n, contrasts = TRUE)
contr.treatment(n, base = 1, contrasts = TRUE)
Arguments
n |
a vector of levels for a factor, or the number of levels. |
contrasts |
a logical indicating whether contrasts should be computed. |
base |
an integer specifying which group is considered the baseline group. |
Details
These functions are used for creating contrast matrices for use in
fitting analysis of variance and regression models. The columns of
the resulting matrices contain contrasts which can be used for coding
a factor with n levels. The returned value contains the
computed contrasts. If the argument contrasts is FALSE
then an additional column of ones is prepended to the matrix.
Note that as from R version 0.62.2, contr.poly returns
contrasts based on orthogonal (rather than raw) polynomials.
Value
A matrix with n rows and k columns, with k=n-1 if
contrasts is TRUE and k=n if contrasts is
FALSE.
See Also
contrasts,
C,
and
aov,
glm,
lm.
Examples
(cH <- contr.helmert(4))
apply(cH, 2,sum)# column sums are 0!
crossprod(cH)# diagonal -- columns are orthogonal
(cT <- contr.treatment(5))
all(crossprod(cT) == diag(4))# TRUE: even orthonormal
(cP <- contr.poly(3))# Linear and Quadratic
zapsmall(crossprod(cP), dig=15) # orthonormal up to fuzz