If the location term in a regression model does not depend on any
previously observed values of the outcome in a longitudinal data set,
then we obtain simplification in our estimation procedure when the
outcomes can be missing. Using the default value of unity for the
argument modify in the mreg function does this.
unity(x, y, mod.Z)A vector of zeroes the same length is the argument x.
is a vector of possible values the previous value of the response could take.
is an vector of the coefficients. Its length is specified in
the mreg function by the argument modify.p.
vector of observed covariates that may interact with the
unobserved preceding outcome corresponding to the observation. It is
taken from the model.frame produced by the mod.formula
argument in mreg.
This function is the default value for the argument modify for
mreg. It does nothing to the linear predictor term.
For this function unity there are no such covariates.
A default value for mod.formula is ~1.