  A study of general properties of the narrow-width approximation (NWA) with
polarization/spin decorrelation is presented. We prove for total rates of
arbitrary resonant decay or scattering processes with an on-shell intermediate
state decaying via a cubic or quartic vertex that decorrelation effects vanish
and the NWA is of order Gamma. Its accuracy is then determined numerically for
all resonant 3-body decays involving scalars, spin-1/2 fermions or vector
bosons. We specialize the general results to MSSM benchmark scenarios.
Significant off-shell corrections can occur - similar in size to QCD
corrections. We qualify the configurations in which a combined consideration is
advisable. For this purpose, we also investigate process-independent methods to
improve the NWA.
