Causal Loop Diagram
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Introduction
Causal Loop Diagrams (CLDs) are structural pictures used to convey understanding about the interactions, or influences, within a structure. A CLD is used to explicity show the nature of the influence relations between the elements of a structure.
There are two main conventions for representing CLDs, one using "+" and "-" and another using "S" and "O" as indicators on the influence from one element to another. The two conventions are relatively similar though when using the "S" and "O" convention there are times when the implications of the diagram can be a bit misleading.
- A --> B with a "+" on the arrow indicates that "A" adds to "B". If "A" increases it adds even more to "B". If "A" decreases is adds less readily to "B" though it still adds.
- A --> B with a "-" on the arrow indicates that "A" subtracts from "B". If "A" decreases is subtracts even more readily to "B". If "A" increases is still subtracts from "B" though not as readily.
- A --> B with an "S" on the arrow indicates that "B" changes in the same direction as "A". If "A" increases "B" increases and if "A" decreases then "B" decreases.
- A --> B with an "O" on the arrow indicates that "B" changes in the opposite direction as "A". If "A" increases then "B" will decrease and if "A" decreases "B" will increase.
- A --> B with no lable indiates that "A" is considered a constant within the boundary of the CLD being considered.
Note there is no indication in a CLD as to the strength of the relation between elements, simply an indication as to the nature of the influence.
Below is provided a description of the Reinforcing Loop and the Balancing Loop. These are the two simplest CLDs one can draw. All other CLDs are, no matter how complicated they may appear, simply combinations of some number of these two loops.
Reinforcing Loop
A reinforcing loop is one in which the interactions are such that each action adds to the other. Any situation where action produces a result which promotes more of the same action is representative of a reinforcing loop.
Fig 1 indicates what happens in a typical savings account. The principal in the savings account interacts with the interest rate and adds to the interest. Note that interest rate is considered to be a constant in this example. Interest then adds to the principal. This reinforcing action happens every so many months depending on the period over which the institution computes the interest. The snowball rolling down hill is your signal that the loop is a reinforcing loop. The small graph to the right of principle indicates that the growth of principal is exponential.
Typical examples of reinforcing loops are population growth and decline, uncontrolled nuclear reactions, snow balls rolling down hill of course, runs on banks, wall street market crashes, etc.
Balancing Loop
A balancing loop is one in which action attempts to bring two things to agreement. Any situation where one attempts to solve a problem or achieve a goal or objective is representative of a balancing loop.
Fig 2 provides the basic form of the balancing loop. The desired state interacts with the current state to produce a gap. The gap adds to the action and the action adds to the current state. The current state then subtracts from the gap. The small clock to the right of the arc between action and current state indicates some time delay that it takes for the action to change the current state. As the current state gets closer to the desired state the gap gets smaller and smaller so it adds less and less to the action, which is adding to the current state. Once the action has moved the current state to a point where it equals the desired state the gap is zero and there's no more addition to the action, so there is no more action. The balance in the center of the loop is your indication that the loop is a balancing loop.
Typical examples of balancing loops are driving from location A to location B, developing a skill, building something, fixing a problem, etc.
Telling One Loop From Another
Initially you might consider it difficult to figure out one loop from the other, yet it's simply a matter of counting. All you need to do is count the number of minus signs around the loop. If there is an even number, or zero, minus signs then it is a reinforcing loop. If there is an odd number of minus signs then it's a balancing loop.
See Also
References
- Bellinger, G. R. 2009. Systems Thinking
- Kim, D. Guidelines for Drawing Causal Loop Diagrams The Systems Thinker. Pegasus Communications.
- Lane, D.C. 2008. The Emergence and Use of Diagramming in System Dynamics: A Critical Account. Systems Research and Behavioral Science Syst. Res. 25, 3-23
- Pegasus Communications. Causal Loop Diagrams
- Richardson, G. P. Problems in Causal Loop Diagrams Revisited
