You enter Reality Check equations in the same way you enter other equations in Vensim. You can use the Sketch Tool to define inputs to Reality Check equations, or simply write the equations directly in the Equation Editor or Text Editor. Structurally, Reality Check equations are not inputs to any model equations, but use model variables in their definitions. When Reality Check equations are exercised, they can change the value of model variables as well as the equations used to compute those variables. Again, we emphasize that Reality Check equations are not statements about causal structure but simply statements about behavior—"If this happens, then that must happen".
The appropriate naming conventions for Reality Check equations are different from those for model variables. Model variables should be named as nouns or noun phrases which more and less have obvious meaning—Workforce, productivity, capacity, determination, propensity to save and so on. Reality Check equations, on the other hand, should be brief phrases that describe the nature of the check—no workers no production, rain means flooding and so on. The best guide in naming Reality Check equations is to think of them as true or false, and name the Reality Check the statement that would be made when it took on a true value.
There are two types of equations that can be defined in Vensim to make use of the Reality Check functionality—Constraints and Test Inputs. Constraints make statements about the consequences that should result from a given set of conditions. They are called Constraints because they specify the way in which the Test Inputs should constrain behavior. The violation of a Constraint indicates a problem with the model. Test Inputs are a way of specifying the conditions or circumstances under which a Constraint is binding. Since Test Inputs can be used in Constraints, they are described first.
