  Evolutionary synthesis models are a fundamental tool to interpret the
properties of observed stellar systems. In order to achieve a meaningful
comparison between models and real data, it is necessary to calibrate the
models themselves, i.e. to evaluate the dispersion due to the discreteness of
star formation as well as the possible model errors. In this paper we show that
linear interpolations in the log M - log t_k plane, that are customary in the
evaluation of isochrones in evolutionary synthesis codes, produce unphysical
results. We also show that some of the methods used in the calculation of
time-integrated quantities (kinetic energy, and total ejected masses of
different elements) may produce unrealistic results. We propose alternative
solutions to solve both problems. Moreover, we have quantified the expected
dispersion of these quantities due to stochastic effects in stellar
populations. As a particular result, we show that the dispersion in the 14N/12C
ratio increases with time.
