  Since a joint sharp measurement of position and momentum is impossible in quantum mechanics, these marginal distributions  (31)  obtained from \(M\) will differ from that of ideal measurements of \(\bQ\) and of \(\bP\) on the system of interest in state \(\ket{\psi}\). However, one can indicate how much these marginals deviate from separate exact position and momentum measurements on the state \(\ket{\psi}\) by a pairwise comparison of  (31)  to the exact distributions 