System DynamicsWhat is System Dynamics? The field of System Dynamics covers a wide ground, but is based on simulation (using a computer) of models which usually contain multiple positive and negative feedback loops. The idea of feedback is extremely important as a way to describe ongoing change in the world. For instance, without feedback, you would not be able to drive a car on the road. Without your eyes seeing the car drift to one side of the road, you could not correct the wheel (the feedback) required to change the course of the car back to the middle of the road. This feedback cannot happen instantly—it only can occur after a period of time. Hence the term dynamic system. A wonderful quote by William S. Bonnell describes the idea of dynamic modeling: "System Dynamics: Things today are the things of yesterday plus any changes. The changes are the result of the things of yesterday. Now extend this to tomorrow." Example ModelThis is a model of the human body growing tolerant to caffeine, a nervous system stimulant. This model is distributed with the free (for personal and educational learning) simulation software Vensim PLE. The interesting thing about dynamic systems is their often counter-intuitive nature. You might think that you would need to drink only 20 percent more coffee to combat 20 percent more stress and fatigue. However, read on to see what actually happens! 
This model describes a short-acting negative feedback loop (goal seeking, or controlling loop) in which raising the amount of Caffeine in Body lowers drowsiness, thereby lowering caffeine consumption, and which eventually lowers the Caffeine in Body (after a short period of time). Unfortunately, a longer-acting positive feedback loop increases the body's need for caffeine. The greater the level of Caffeine in Body, the more the level of Body's Expected Caffeine (after a longer period of time—the caffeine tolerance time) resulting in more drowsiness, more caffeine consumption, and more Caffeine in Body again, completing the positive feedback. Here is a graph of caffeine consumption after a rise in stress causes a 20 percent increase in your coffee drinking. 
The result is an eventual 100 percent increase (double the amount of coffee drinking) to cope with the small (20 percent) increase in stress! Links to websites on System DynamicsYou can learn more about System Dynamics at many web sites, including:
Copyright James Melhuish 1999 - 2007
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