.\" -*- nroff -*- generated from .Rd format
.BG
.FN varExp
.TL
Exponential Variance Function
.DN
This function is a constructor for the `varExp' class, representing an
exponential variance function structure. Letting v denote the variance
covariate and s2(v) denote the variance function evaluated at v, the
exponential variance function is defined as s2(v) = exp(2 * t * v),
where t is the variance function coefficient. When a grouping factor
is present, a different t is used for each factor level.
.CS
varExp(value, form, fixed)
.OA
.AG value
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).
.AG form
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 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.
.AG fixed
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.
.RT
a `varExp' object representing an exponential variance function
structure, also inheriting from class `varFunc'.

.SA
`varWeights.varFunc', `coef.varExp'
.EX
vf1 <- varExp(0.2, form = ~age|Sex)
.KW models
.WR
