Terminating Versus
Non-terminating Simulations

As part of setting up the simulation experiment, one must decide what type of simulation to run. Simulations are usually distinguished as being one of two types: terminating or non-terminating. The difference between the two has to do with whether we are interested in the behavior of the system over a particular period of time or in the steady-state behavior of the system. It has nothing to do, necessarily, with whether the system itself terminates or is ongoing. The decision to perform a terminating or non-terminating simulation has less to do with the nature of the system than it does with the behavior of interest.

A terminating simulation is one in which the simulation starts at a defined state or time and ends when it reaches some other defined state or time. An initial state might be the number of parts in the system at the beginning of a work day. A terminating state or event might be when a particular number of jobs have been completed. Consider, for example, an aerospace manufacturer that receives an order to manufacture 200 airplanes of a particular model. The company might be interested in knowing how long it will take to produce the aircraft along with existing workloads. The simulation run starts with the system empty and is terminated when the 200th plane is completed since that covers the period of interest. A point in time which would bring a terminating simulation to an end might be the closing of shop at the end of a business day, or the completion of a weekly or monthly production period. It may be known, for example, that a production schedule for a particular item changes weekly. At the end of each 40 hour cycle, the system is "emptied" and a new production cycle begins. In this situation, a terminating simulation would be run in which the simulation run length would be 40 hours.

Terminating simulations are not intended to measure the steady-state behavior of a system. In a terminating simulation, average measures are of little meaning. Since a terminating simulation always contains transient periods that are part of the analysis, utilization figures have the most meaning if reported for successive time intervals during the simulation.

A non-terminating or steady-state simulation is one in which the steady-state behavior of the system is being analyzed. A non-terminating simulation does not mean that the simulation never ends, nor does it mean that the system being simulated has no eventual termination. It only means that the simulation could theoretically go on indefinitely with no statistical change in behavior. For non-terminating simulations, the modeler must determine a suitable length of time to run the model. An example of a non-terminating simulation is a model of a manufacturing operation in which oil filters are produced on a continual basis at the same pace. The operation runs two shifts with an hour break during each shift in which everything momentarily stops. Break and third shift times are excluded from the model since work always continues exactly as it left off before the break or end of shift. The length of the simulation is determined by how long it takes to get a representative steady-state reading of the model behavior.