P ( a n ) = | ⟨ ⟨ a n | ψ ψ ⟩ ⟩ | 2 (Discrete, nondegenerate spectrum) P ( a n ) = ∑ ∑ i g n | ⟨ ⟨ a n i | ψ ψ ⟩ ⟩ | 2 (Discrete, degenerate spectrum) d P ( α α ) = | ⟨ ⟨ α α | ψ ψ ⟩ ⟩ | 2 d α α (Continuous, nondegenerate spectrum)