| 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. Ignored if |
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 a square indicator matrix is returned.
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
contr.helmert(4, contrasts = FALSE) # just the 4 x 4 identity matrix
(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