  A formal derivation of the polarization correlations between the incident
electron and the scattered electron is given for a general class of transition
operators. In correspondence to the case of bremsstrahlung emission, three sum
rules for the polarization correlations are predicted, which reduce to the
known one for potential scattering. Further examples, including the 1_2^+ and
2_1^+ excitations of the 12C nucleus and a 1^- excitation of the 208Pb nucleus,
are discussed and the validity of the corresponding sum rules is investigated.
