varIdent | R Documentation |
This function is a constructor for the varIdent
class,
representing a constant variance function structure. If no grouping
factor is present in form
, the variance function is constant
and equal to one, and no coefficients required to represent it. When
form
includes a grouping factor with M > 1
levels, the
variance function allows M different variances, one for each level of
the factor. For identifiability reasons, the coefficients of the
variance function represent the ratios between the variances and a
reference variance (corresponding to a reference group
level). Therefore, only M-1
coefficients are needed to represent
the variance function. By default, if the elements in value
are
unnamed, the first group level is taken as the reference level.
varIdent(value, form, fixed)
value |
an optional numeric vector, or list of numeric values,
with the variance function coefficients. If no grouping factor is
present in |
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. It must have names
identifying which coefficients are to be fixed. Coefficients
included in |
a varIdent
object representing a constant variance function
structure, also inheriting from class varFunc
.
JosÃ© 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.
varClasses
,
varWeights.varFunc
,
coef.varIdent
vf1 <- varIdent(c(Female = 0.5), form = ~ 1 | Sex)
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