Feedback
From WikiSD
When an action turns out to have an influence on itself, no matter how many steps removed that might be involved, that influence is defined as feedback. Based on the types of interactions defined in Systems Thinking feedback can have a positive or negative effect. That is, the feedback can add to, "+", the initial action, or it can subtract from, "-", the action.
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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. This is an example of positive, "+", feedback.
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. As such, it is the positive feedback from principal to interest that is responsible for promoting growth within this structure.
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. This is an example of negative, "-", feedback.
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. As such, it is the negative feedback that actually reduces the gap to zero thus eliminating the basis for continued action.
Typical examples of balancing loops are driving from location A to location B, developing a skill, building something, fixing a problem, etc.
See Also
References
- Wikipedia. Feedback
- Wikipedia. Negative Feedback
- Wikipedia. Positive Feedback
