  General statistical ensembles in the Hamiltonian formulation of hybrid
quantum-classical systems are analyzed. It is argued that arbitrary probability
densities on the hybrid phase space must be considered as the class of possible
physically distinguishable statistical ensembles of hybrid systems.
Nevertheless, statistical operators associated with the hybrid system and with
the quantum subsystem can be consistently defined. Dynamical equations for the
statistical operators representing the mixed states of the hybrid system and
its quantum subsystem are derived and analyzed. In particular, these equations
irreducibly depend on the total probability density on the hybrid phase space.
