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You build and use models in order to address problems.  As these models are built, there are various checks that must be done against reality.  These checks may be explicit and take the form of tests of model behavior or subsector behavior under different assumptions, or they may be implicit mental simulations and analysis based on your own understanding of models and the modeling process.  In either case these checks are very important in ensuring that the models you develop can adequately address the problems they are being applied to.

Reality Check gives you a straightforward way to make statements you think must be true about a model for it to be useful, and the machinery to automatically test a model for conformance with those statements.  Reality Check is a new technology that adds significantly to your ability to validate and defend the models you build.  It can also focus discussion away from specific assumptions made in models onto more solidly held beliefs about the nature of reality.

This chapter:

Introduces the concept of a Reality Check.
Shows you how to define Constraints and Test Inputs.
Shows you how to test a model for conformance to Reality Check equations.
Provides a simple example of model building using Reality Check.