An object of class survival returned from Psm$sim_survival() or
PsmCurves$survival().
A survival object inherits from data.table and contains
the following columns:
A random sample from the PSA.
The treatment strategy ID.
The patient ID.
The subgroup ID.
One of the N-1 survival curves in an N-state partitioned
survival model. Each curve corresponds to unique endpoint.
The time at which a survival probability is computed.
The probability of surviving to time t.