varConstPower
class,
representing a constant plus power variance function
structure. Letting v denote the variance covariate and
s2(v) denote the variance function evaluated at
v, the constant plus power variance function is defined as
s2(v) = (t1 + |v|^t2)^2,
where t1, t2 are the variance
function coefficients. When a grouping factor is present, different
t1, t2 are used for each factor level.
varConstPower(const, power, form, fixed)
form
. If either argument has
length greater than one, it must have names which identify its
elements to the levels of the grouping factor defined in
form
. If a grouping factor is present in
form
and the argument has length one, its value will be
assigned to all grouping levels. Only positive values are allowed
for
const
. 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 must evaluate to a
numeric vector and may involve expressions using
"."
, representing a
fitted model object from which fitted values (
fitted(.)
) and
residuals (
resid(.)
) can be extracted (this allows the variance
covariate to be updated during the optimization of an objective
function). When a grouping factor is present in
form
, a different
coefficient value is used for each of its levels. 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 `~ fitted(.)'
representing a variance covariate given by the fitted values of a
fitted model object and no grouping factor.
const
and/or
power
, consisting of numeric vectors, or lists of numeric
values, specifying the values at which some or all of the
coefficients in the variance function should be fixed. If a grouping
factor is specified in
form
, the components of
fixed
must have names identifying which coefficients are to be
fixed. Coefficients included in
fixed
are not allowed to vary
during the optimization of an objective function. Defaults to
NULL
, corresponding to no fixed coefficients.
varConstPower
object representing a constant plus power
variance function structure, also inheriting from class
varFunc
.
You can use the functions
varWeights.varConstPower
and
coef.varConstPower
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 <- varConstPower(1.2, 0.2, form = ~age|Sex)