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## Main Question or Discussion Point

According to one of the postulates of quantum mechanics, every measured observable q is an eigenvalue of a corresponding linear Hermitian operator Q. Which means, that q must satisfy the equation Qψ = qψ. But according to Griffiths chapter 3, this equation can only be followed from σ

_{Q = 0. It makes no sense to me because not every observable has zero standard deviation. Can someone explain this?}