Before we discuss the mechanics of actually running Reality Check equations, it is useful to describe very briefly what happens inside the model. Reality Check equations involve systematic intervention in the basic structure of the model. They are qualitatively different from sensitivity analysis in that there are not any well defined pathways of influence. Test Inputs and Constraints can cause changes to be made at almost any point in a model.
In order to accomplish the changes involved in running Reality Check equations, Vensim restructures your model, adding equations and modifying the sequence in which equations are computed to match. After completing Reality Check equations, Vensim returns the model to its original structure. This means that doing causal tracing on a run made with one or more Test Inputs active can give surprising and seemingly incorrect results.
In some cases, restructuring and reordering may leave the model ill formed. The most common problem is that the model may contain simultaneous equations and therefore not be computable. If this is the case, Vensim will report the problem and not complete the simulation. Because of the behavior-behavior nature of Reality Check equations, the existence of simultaneous equations does not necessarily represent a conceptual problem, but will prevent simulation. If possible, reformulate the Test Inputs causing the problems to avoid simultaneous equations.
NOTEReality Check equations are always run using interpreted (not compiled) simulations. This is because they require continual restructuring of equations.
