Measurement of the observable A yields an eigenvalue of A. Repeated measurements, made on a statistical ensemble S of systems, results in a probability distribution over the eigenvalue spectrum of A. It is a discrete probability distribution, and is given by Pr ⁡ ⁡ ( λ λ ) = Tr ⁡ ⁡ ( S E A ⁡ ⁡ ( λ λ ) ) .