varConstPower {nlme} | R Documentation |

This function is a constructor for the `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)

`const, power` |
optional numeric vectors, or lists of numeric
values, with, respectively, the coefficients for the constant
and the power terms. Both arguments must have length one, unless a
grouping factor is specified in `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). |

`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 object
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, as
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. |

`fixed` |
an optional list with components `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. |

a `varConstPower`

object representing a constant plus power
variance function structure, also inheriting from class
`varFunc`

.

Jose Pinheiro Jose.Pinheiro@pharma.novartis.com and Douglas Bates bates@stat.wisc.edu

`varWeights.varFunc`

, `coef.varConstPower`

vf1 <- varConstPower(1.2, 0.2, form = ~age|Sex)

[Package *nlme* version 3.1-66 Index]