Models and simulations

A model is a simplified copy of something real. A road map keeps roads and names and leaves out trees and shop windows, because the purpose of the map is to find the way. A model is useful because it is simpler than the real thing. We can sort models in two ways. Static models do not change with time (a map, a recipe for four people), and dynamic models change with time (the growth of savings over the years, a weather forecast). Deterministic models give the same result every time you give them the same input (a formula that adds two prices); probabilistic models include chance, so the results vary from run to run (rolling a die in a board-game model). Making a model has steps: state the purpose, choose the facts that matter, build the model, test it against reality, and improve it. If the test shows that the model is wrong, we do not blame reality: we go back to the choice of facts. A model is never the whole truth, but a good model is good enough for its purpose.