getDDFM: Degrees of Freedom for Testing Linear Contrast of Fixed Effects and Least Square Means.
Description
There are three methods implemented, which are all
available in SAS PROC MIXED, namely "contain", "residual",
and "satterthwaite" approximations. See the documentation
of SAS PROC MIXED for details on this topic.
(numeric) vector specifying the linear
combination of the fixed effect or LS Means
ddfm
(character) string specifying the method used
for computing the denominator degrees of freedom for
tests of fixed effects or LS Means. Available methods are
"contain", "residual", and "satterthwaite".
tol
(numeric) value specifying the numeric
tolerance for testing equality to zero
method.grad
(character) string specifying the
method to be used for approximating the gradient of the
variance-covariance matrix of fixed effects at the
estimated covariance parameter estimates (see function
'grad' (numDeriv) for details)
opt
(logical) TRUE = tries to optimize computation
time by avoiding unnecessary computations for balanced
datasets (see test.fixef).
items
(list) of pre-computed values
Value
(numeric) vector with the specified type of degrees of
freedom
Details
The implementation of the Satterthwaite approximation was
inspired by the code of function 'calcSatterth' of
R-package 'lmerTest'.