A Population Model
This chapter features a simulation model of rabbit population. The modeling process starts with sketching a model, then writing equations and specifying numerical quantities. Next, the model is simulated with simulation output automatically saved as a dataset. Finally, the simulation data can be examined with Analysis tools to discover the dynamic behavior of variables in the model.
Normal model construction follows a pattern of create, examine, and recreate, iterating until your model meets your requirements. Debugging (making a model simulate properly) and model analysis (investigating output behavior) both play a part in refining the model. Reality Check is another technology to aid in the construction and refinement of models and is described in Chapter 14.
The behavior of a simulation model in Vensim is solely determined by the equations that govern the relationships between different variables. We will list those equations in full for the simulation models developed in this Guide. The diagram of a model (causal loop or stock and flow) is a picture of the relationships between variables. Vensim enforces consistency of the diagram and model equations, but information can be omitted or hidden in diagrams. When you are building a simulation model, make sure the equations match what is in this manual. If there is a discrepancy in the appearance of the diagrams, it may be because you have inadvertently hidden something or set an option that influences the appearance of model diagrams.