Quantum Time Evolution in Terms of Nonredundant Expectation Values

Each scheme of state reconstruction comes down to parametrize the state of a quantum system by expectation values or probabilities directly measurable in an experiment. It is argued that the time evolution of these quantities provides an unambiguous description of the quantal dynamics. This is shown explicitly for a single spin s, using a quorum of expectation values which contains no redundant information. The quantum mechanical time evolution of the system is rephrased in terms of a closed set of linear first-order differential equations coupling (2s+1)^2 expectation values. This `realization' of the dynamical law refers neither to the wavefunction of the system nor to its statistical operator.