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Unformatted text preview: 41 4 Causal Loop Diagramming Vensim Models This chapter describes causal loop. Causal loop diagrams are called that because each link has a causal interpretation. An arrow going from A to B indicates that A causes B. Causal loop diagrams can be very helpful in conceptualizing and communicating structures. Many people find causal loop diagramming to be very helpful even when no simulation model is created, while others feel they can be harmful if done in isolation. This chapter is primarily about technique, and is useful to work through even if you choose not to build any additional causal loop diagrams. Causal loop diagrams are also often called influence diagrams. Causal loop diagrams do not show accumulations (levels or stocks) in a system. Construction of stock and flow diagrams is covered in Chapter 5. However, even if you intend to build only stock and flow diagrams, we recommend that you start with this chapter as many of the basic drawing mechanics are the same and they are covered in more detail here. It is important to note that causal loop diagrams and stock and flow diagrams are not simulation models. Simulation models, like the one used in Chapter 3, attach algebraic relationships to all the variables appearing in a diagram. In Chapter 6, "Building a Simulation Model" we describe how to create a simulation model. If you are using Vensim Professional or DSS you can skip building model diagrams altogether enter equations directly. Almost all people, however, find it easier to build up models diagrammatically. Drawing Sketches When a Sketch Tool is selected, that tool remains active until you select another tool. A single click (press and release) with the mouse button applies the tool to the sketch. The Lock tool provides the standard mouse cursor. The Lock tool can be used for selecting sketch objects (they highlight black) and for changing options. Sketch objects cannot be moved with the Lock tool. Tip you can select the Lock tool by pressing the Esc key, or the keyboard number 1. The Move/Size tool is used for moving sketch objects around, including resizing variables and boxes, and reshaping arrows. The other sketch tools also allow you to move objects. Variable sketch tools ( Variable Box Variable and other variable tools you may configure) and the default setting for the Rates tool bring up editing boxes (for naming the Variable or Rate) when applied to the sketch. The Sketch Comment tool brings up a dialog box. 4: Vensim Users Guide 42 The Arrow tool starts an arrow. To do this make a single click (press and release) of the mouse button on the middle of the starting word, then finish with another single click on the middle of the ending word. Arrows (curved) can take one intermediate point on a sketch with an extra mouse click....
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- Fall '08