In this case, the probability that the energy value measured for a system in the state | ψ ψ ⟩ ⟩ will yield the value E n is given by the sum of the probabilities of finding the system in each of the states in this basis, i.e., P ( E n ) = ∑ ∑ i = 1 g n | ⟨ ⟨ E n , i | ψ ψ ⟩ ⟩ | 2