An object of class stateprobs returned from methods
$sim_stateprobs() in model classes.
A stateprobs object inherits from data.table and contains
the following columns:
A random sample from the PSA.
The treatment strategy ID.
The patient ID.
The health state ID.
The time at which a state probability is computed.
The probability of being in a given health state.
When simulating individual-level models, the patient_id column is
not included as state probabilities are computed by averaging across patients.