contrast {base} | R Documentation |
Return a matrix of contrasts.
contr.helmert(n, contrasts = TRUE) contr.poly(n, contrasts = TRUE) contr.sum(n, contrasts = TRUE) contr.treatment(n, base = 1, contrasts = TRUE)
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. |
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.
A matrix with n
rows and k
columns, with k=n-1
if
contrasts
is TRUE
and k=n
if contrasts
is
FALSE
.
contrasts
,
C
,
and
aov
,
glm
,
lm
.
(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