Assigning polarity to flows and connectors in your model allows you to visually indicate the cause-and-effect relationship between model entities. Assigning polarity can also make it easier to tell a story about the feedback loops in your model.
For example, in the following model, when being born increases, its effect is to also increase Population (note the "+" polarity sign at the right end of the being born flow). This is a positive polarity. Conversely, as dying increases, the effect is to decrease Population, a negative ("-") polarity.
An odd number of -/o links around a loop signifies a Balancing loop. An even number of -/o links signifies a Reinforcing loop.
For connectors, you can choose the polarity you want to assign (positive or negative), based on the interaction between the two entities.
For flows, the polarity is determined by the flow's association to the attached stocks: an inflow always has a positive (+/s) influence on a stock and an outflow always has a negative (-/o) influence on a stock. Although you can't select their polarity, you can choose whether or not to display their polarity in the model.
Note: The default polarity symbols that are displayed on the diagram are - and +. If you prefer to use the letter symbols (o and s) instead, select the Use Lettered Polarity check box in the Model Preferences dialog box.
See Also