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. Ignored if contrasts is FALSE . |
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.
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 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