When System Dynamics ODE Models Fail

Two examples of models formulated in terms of the Ordinary Differential Equations (ODE) are given. It is pointed out that, in certain situations, the ODE- based models commonly used in system dynamics may be completely wrong. Even in some very simple models of the dynamics of populations it is not certain that any finite state vector of the system exists, which means that the corresponding finite ODE model does not exist. If the modeled system is nonlinear, the existence of solutions to the ODE model is important. The mapping from the space of disturbances to the space of corresponding trajectories may be discontinuous, even if the right-hand sides of the equations are continuous. This may cause errors when simplified models are used. A possible application of the discrete event and object-oriented simulation instead of the ODE models is suggested.