Before proceeding with the details of working with a sketch, it is important to distinguish between Defined variables and Shadow variables. This distinction has nothing to do with the model or logical interconnections, but only with the visual character of a sketch. When working with views, however, the distinction is important.
Defined Variables
A variable is defined within a view if all of its causes are shown with arrows coming from them into the variable. Vensim that makes sure that all of the causes are shown. If you add or remove arrows in one view, Vensim will do the same in other views. Similarly, if you change the logical connection in equations, Vensim will change the sketch to correspond. Thus, it is not possible to include only some of the inputs to a Defined variable. You can, however, hide some of the causes of a Defined variable to get the same visual effect.
A variable can only appear as a Defined variable once in a view, although a Defined variable can appear elsewhere in the view as a Shadow variable. One variable can appear as a Defined variable in any number of different views.
Defined variables are displayed in the sketch as just the name of the variable, and have arrows coming from their causes. If a variable has no causes (such as a Constant), it can still be recognized as a Defined variable because it lacks the angle brackets < > that are used to mark Shadow variables.
Shadow Variables
A Shadow variable is a variable that is defined elsewhere. No arrows can enter (cause) Shadow variables. Arrows can leave Shadow variables. Shadow variables refer to variables defined elsewhere in a view, in other views or in an equation and are useful in helping to reduce clutter and increase the clarity of a sketch. Note that though Shadow variables do refer to variables defined elsewhere, the associated Defined variable need not appear in the current or any view. Vensim lets you mix and match visual representations with equations having no visual counterpart.
Shadow variables appear on a sketch surrounded by angle brackets < >. Thus Population would appear as <Population>. By default Shadow variables are shown in gray, although you can change their color if you want.
Shadow variables are simply a convenient substitute for Defined variables, and indicate that you need to look elsewhere to find the definition. They are neither a different variable nor a different type of variable from a Defined variable.
