Exponential Variance Function
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.
varExp(value, form, fixed)
value |
an optional numeric vector, or list of numeric values,
with the variance function coefficients. |
form |
an optional one-sided formula of the form |
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 |
a varExp
object representing an exponential variance function
structure, also inheriting from class varFunc
.
José Pinheiro and Douglas Bates bates@stat.wisc.edu
Pinheiro, J.C., and Bates, D.M. (2000) "Mixed-Effects Models in S and S-PLUS", Springer.
vf1 <- varExp(0.2, form = ~age|Sex)
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