Thinking in Systems
Complex systems get built up by similar simple patterns. Understanding each pattern helps understand a more complicated system.
A two balanced loop system occurs when there are two loops trying to drag a system in opposing direction. A good example is a thermostat that’s trying to keep a house at a certain temperature while the cold outside air is draining the house of heat.
Feedback loops have an inherit delay that allows them only effect the future value of the stock. This mechanism prevents a loop in a competing system from reacting quickly enough to reach it’s desired set-point precisely.
A system with a competing reinforcing loop and balancing loop can only toggle between the two. When the reinforcing loop dominates the system, the stock will grow or fall exponentially. When the balancing loop dominates the stock will grow or fall linearly. Population represents this type of system with the birth rate as the reinforcing loop, and the death rate as the balancing loop.
Test the value of a model by asking:
- Are the driving factors likely to unfold this way?
- If they did, would the system react this way?
- What is driving the driving forces?
Most often, the driving forces of a system will have other forces driving them.
Even systems with vastly different exteriors will behave the similarly if they are driven by the same feedback loops. Population and Industrial Economy provide an example of this phenomena.
Many real world systems have further external delays in their feedback loops. If these are not managed properly the stock oscillates out of control.
Controlling any single delay can have profound effects on the entire system; sometimes, in counter intuitive ways.