varIdent
class,
representing a constant variance function structure. If no grouping
factor is present in
form
, the variance function is constant
and equal to one, and no coefficients required to represent it. When
form
includes a grouping factor with M > 1 levels, the
variance function allows M different variances, one for each level of
the factor. For identifiability reasons, the coefficients of the
variance function represent the ratios between the variances and a
reference variance (corresponding to a reference group
level). Therefore, only M-1 coefficients are needed to represent
the variance function. By default, if the elements in
value
are
unnamed, the first group level is taken as the reference level.
varIdent(value, form, fixed)
form
, this argument is ignored, as the resulting
variance function contains no coefficients. If
value
has
length one, its value is repeated for all coefficients in the
variance function. If
value
has length greater than one, it
must have length equal to the number of grouping levels minus one
and names which identify its elements to the levels of the grouping
factor. Only positive values are allowed for this argument. Default
is
numeric(0)
, which results in a vector of zeros of
appropriate length being assigned to the coefficients when
object
is initialized (corresponding to constant variance
equal to one).
v
and, optionally, a grouping factor
g
for the coefficients. The variance covariate is ignored in this
variance function. When a grouping factor is present in
form
, a
different coefficient value is used for each of its levels less one
reference level (see description section below). Several grouping
variables may be simultaneously specified, separated by the
*
operator, like in `~ v | g1 * g2 * g3'. In this case, the levels of
each grouping variable are pasted together and the resulting factor is
used to group the observations. Defaults to `~ 1'.
fixed
are not allowed to vary during the
optimization of an objective function. Defaults to
NULL
,
corresponding to no fixed coefficients.
varIdent
object representing a constant variance function
structure, also inheriting from class
varFunc
.
You can use the functions
varWeights.varIdent
and
coef.varIdent
to extract the weights and coefficients, respectively, from the variance function structure. For more details, see the links below for the generic functions
varWeights
and
coef.varFunc
.
vf1 <- varIdent(c(Female = 0.5), form = ~ 1 | Sex)