- A
*formal model*is a precise statement of components to be used and the relationships among them. Formal models are usually stated via mathematics, often equations. - Formal models can be precisely communicated because they are well-defined.
- Formal models give replicable results. This is the simple meaning of "mathematical proof".
- Formal models are
*not*reality: you must*choose*the model. - Formal models may not correspond to reality---the prediction will turn out to be false.
- Informal models simply have some lack of precision. Some relationships may not be stated as equations, or "expert judgment" will modify the result of the model.