  We present a method to analyze the results of first-principles based
calculations of electronic currents including inelastic electron-phonon
effects. This method allows us to determine the electronic and vibrational
symmeties in play, and hence to obtain the so-called propensity rules for the
studied systems. We show that only a few scattering states -- namely those
belonging to the most transmitting eigenchannels -- need to be considered for a
complete description of the electron transport. We apply the method on
first-principles calculations of four different systems and obtain the
propensity rules in each case.
