A feedback loop is formed when changes in a stock affect the flows into or out of that stock. It is kind of a control mechanism that affects and gets affected by the stock level at that point in time. Feedback loops can cause stocks to maintain their level within a range, grow or decline.
The standard definition can be qouted as, “A feedback loop is a closed chain of causal connections from stock, through a set of decisions or rules or physical laws or actions that are dependent on the level of the stock, and back again through a flow to change the stock.” Okay, I know that’s too much to gobble, but worry not, we will go through some simple examples to understand how feedback works and affects the system behavior. In general, there are two kinds of loop - Balancing or Restoring and Reinforcing.
Balancing Feedback Loop
This feedback tends to maintain the stock at some level. It acts as a restoring force or ‘resistance to change’ in the stock level. For instance take a ball inside a bowl, even if the ballis displaced a bit, it is restored to a position such that its gravitational potential energy is minimised. A stock regulated with a balancing feedback loop remains in stable equilibrium and has the capacity to absorb shocks up to a certain threshold.
Does this reminds you of Newton’s first law of motion ‘The inherent property of a body is to resist a change in its motion, is called as inertia and is directly proportional to the mass.’ Yes, by now you must have understood stocks acts as buffers, sources of inertial, resistance to a change in the system externalities which affects the flows. Isn’t it great that Physics and Systems Science go hand in hand? In fact, you will see, physics is in fact a subset of systems science.
Reinforcing Feedback Loop
This feedback tends to increase or decrease the stock level depending on the incident perturbation. If the perturbation tends to increase the stock level, the feedback loop will reinforce this action and increase the stock level further and vice versa. A common example is the stock of population, the higher the population, the higher the births which again causes the population to increase. This reinforcing action causes the population stock to rise. These types of system tend to exhibit unstable equilibrium behaviour. If they are perturbed then due to this reinforcing nature they can’t be restored to their natural balanced state on their own and require some external agent.
By now, hopefully you will be clear of how feedback works. It reacts to a certain quantity internal or external to the system and creates a response which tries to alter the level of the stock. In this example, take the stock as the potential energy of the ball, in the reinforcing feedback loop (inverted bowl) the decrease in potential energy of the ball stimulates the system which accelerates the ball in the direction of motion. This again leads to decrease in potential energy and this happens till the decrease is zero, i.e the ball becomes stable in the direction perpendicular to the direction of gravity (in this case).
Hope you are now familiar with the basics of Systems - Stocks, Flows and Feedbacks. In the next article, we will look at Causal Loop Diagrams, a nice and intuitive to graphically depict the system and gain useful insights. There’s a lot of ground to cover up people! Hope you are enjoying! Stay Tuned!