This chapter describes causal loop diagramming. 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.
A completed example of the model is provided for reference; see project.mdl located in the directory UserGuide\chap04\Complete.
