Finally, the systems we have been talking about are all deterministic. You should know that if the numbers of particles or individuals in a compartment are small, the models for the transfers are really probabilistic. That gives rise to stochastic compartmental models which are much more difficult to solve than the deterministic ones. Whenever the transfers must be of discrete units, the appropriate models are stochastic and the deterministic models are only approximations. Nonetheless, there is much to be learned from the use of the deterministic models.