I red griffiths many times but even now there is something I can't understand. It's about statistical interpretation. In his book chapter 3.4 he says "If you measure an observable Q(x,p) on a particle in the state ψ(x,t), you are certain to get one of the eigenvalues of the hermitian operator Q(x,-ihd/dx)" but when the particle is not in determinate state (I mean <σ^2>=0), we can't even get eigenvalue equation Qψ=qψ. So we don't know whether the observable is a eigenvalue of some eigenvalue equation or not. Could you please explain the sentence above to me?