varPower {nlme} | R Documentation |

This function is a constructor for the `varPower`

class,
representing a power variance function structure. Letting
*v* denote the variance covariate and *s2(v)*
denote the variance function evaluated at *v*, the power
variance function is defined as *s2(v) =
|v|^(2*t)*, where *t* is the variance
function coefficient. When a grouping factor is present, a different
*t* is used for each factor level.

varPower(value, form, fixed)

`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). |

`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, 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. |

`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. |

a `varPower`

object representing a 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.varPower`

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

[Package *nlme* version 3.1-66 Index]