Most of the best policy models are small and simple. At least in their essentials, they can be easily understood and described to others. They present relatively modest computational demands for modern digital computers. These attributes flow directly from the requirements that to be useful, policy models must be understandable and modest enough to be vigorously exercised to explore the implications of alternative assumptions and polices.

Of course, many models are not small and simple. There are some models, especially some science and engineering models, that are large or complex because they need to be. But many more are large or complex because their authors gave too little thought to why and how they were being built and how they would be used.

Policy analysts occasionally find themselves confronted with large and complex models. Sometimes the problem is to use the results or insights gained from a large science or engineering model in a policy analysis. More often the problem involves large and complex models someone wants to use directly in policy analysis. This chapter briefly explores some of the special issues that arise in such situations.

What Are “Large” and “Complex” Models?

Large models are models requiring large amounts of human, computational, or other resources in their construction and operation.