Obtain standard start values.
std_start(cov_type, n_visits, n_groups, ...)A numeric vector of starting values.
(string)
name of the covariance structure.
(int)
number of visits.
(int)
number of groups.
not used.
std_start will try to provide variance parameter from identity matrix.
However, for ar1 and ar1h the corresponding values are not ideal because the
\(\rho\) is usually a positive number thus using 0 as starting value can lead to
incorrect optimization result, and we use 0.5 as the initial value of \(\rho\).