 Ontological model of a quantum system

An ontological model associates each quantum state preparation with a probability distribution over underlying physical states $\lambda$. A quantum measurement's response probabilities depend on its specification and $\lambda$, sum to one, and reproduce the Born rule when averaged over the preparation distribution. The model need not assign deterministic outcomes. Overlap or disjointness of preparation measures defines the psi-epistemic model versus psi-ontic model distinction.