  In molecular photofragmentation processes by soft X-rays, a number of ionic
fragments can be produced, each having a different abundance and correlation
with the emitted electron kinetic energy. For investigating these fragmentation
processes, electron-ion and electron-ion-ion coincidence experiments, in which
the kinetic energy of electrons are analyzed using an electrostatic analyzer
while the mass of the ions is analyzed using a pulsed electric field, are very
powerful. For such measurements, however, the contribution of random
coincidences is substantial and affects the data in a non-trivial way. Simple
intuitive subtraction methods cannot be applied. In the present paper, we
describe these electron-ion and electron-ion-ion coincidence experiments
together with a subtraction method for the contribution from random
coincidences. We provide a comprehensive set of equations for the data
treatment, including equations for the calculation of error-bars. We
demonstrate the method by applying it to the fragmentation of free CF$_3$SF$_5$
molecules.
