If NULL, random effects are simulated from their estimated
distribution for each draw in nsim, i.e. the simulation is from the
marginal/joint distribution of the response (and random effects).
Otherwise the simulation is conditional on the supplied random effects.
When ranef = "zero", a vector of zeros with the right size is
substituted.
what
Defaults to 'response'. 'ranef' returns draws from
the random effects distribution, 'joint' results in simulated data
from the joint distribution of random effects and responses. When it is
set to other than 'response', ranef=NULL and bysim=TRUE must
be set.
bysim
logical, if TRUE a list with nsim elements is returned,
each element is of length nrow(newdata) and
contains one sample from the conditional distribution for each
row of newdata. If FALSE, a list of length nrow(newdata)
is returned, its ith element of length nsim contains nsim samples
from the conditional distribution given newdata[i,].