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
a square indicator matrix (the dummy coding) is returned except
for contr.poly
(which includes the 0-degree, i.e.constant,
polynomial when contrasts = FALSE
).
contr.helmert
returns Helmert contrasts, which contrast the
second level with the first, the third with the average of the first
two, and so on. contr.poly
returns contrasts based on
orthogonal polynomials. contr.sum
uses ‘sum to zero
contrasts’.
contr.treatment
contrasts each level with the baseline level
(specified by base
): the baseline level is omitted. Note that
this does not produce ‘contrasts’ as defined in the standard
theory for linear models as they are not orthogonal to the intercept.
contr.SAS
is a wrapper for contr.treatment
that sets
the base level to be the last level of the factor. The coefficients
produced when using these contrasts should be equivalent to those
produced by many (but not all) SAS procedures.
For consistency, sparse
is an argument to all these contrast
functions, however sparse = TRUE
for contr.poly
is typically pointless and is rarely useful for
contr.helmert
.