value
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an optional numeric vector, or list of numeric values,
with the variance function coefficients. Value must have
length one, unless a grouping factor is specified in form.
If value 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 value has length one, its value will be
assigned to all grouping levels. 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).
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form
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an optional one-sided formula of the form ~ v, or
~ v | g, specifying a variance covariate 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 object
function). When a grouping factor is present in form,
a different coefficient value is used for each of its
levels. Defaults to ~ fitted(.) representing a variance
covariate given by the fitted values of a fitted model object and no
grouping factor.
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fixed
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an optional numeric vector, or list 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, 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.
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