dataframe
the data in the format of a data frame in R that contain a column of subject ID numbers
(i.e., an ID variable), a column indicating subject-specific measurement occasions
(i.e., a TIME variable), at least one column of observed values, and any number of covariates.
If the data are fit to a discrete-time model, the TIME variable should contain subject-specific sequences
of consecutively equally spaced numbers (e.g, 1, 2, 3, ...).
If the data are fit to a continuous-time model, the TIME varibles can contain subject-specific increasing sequences
of irregularly spaced real numbers.
Missing values in the observed variables shoud be indicated by NA. Missing values in covariates are not allowed.
That is, missing values in the covariates, if there are any, should be imputed first.