Systems everywhere

What is a system?

A system is a group of parts that influence each other and together do something. A bicycle is a system: pedals, chain and wheels work together to move you. A class at school is a system too: pupils, teachers, homework and the timetable all affect each other.

The interesting thing about systems is that the whole behaves differently from the parts. One car is not a traffic jam. One rabbit is not a population that grows and shrinks in waves. To understand the jam or the waves, you have to look at how the parts are connected.

System dynamics is a way of drawing those connections and letting a computer play them forward in time. It was invented in the 1950s by Jay Forrester, an engineer at MIT, to understand why factories had ups and downs that nobody wanted. Today it is used for climate, health, cities, companies and school projects.

Stocks pile up

A stock is anything that can pile up or run down: water in a bathtub, money in a piggy bank, sand in an hourglass, friends you have, homework still to do. If you could freeze time and count something, it is a stock.

Stocks have two more habits worth knowing. They change slowly: you cannot fill a bathtub in a second, or make a forest grow overnight. And they remember: what is in the tub today is the sum of everything that came in and went out before.

Flows fill and drain

A flow is what changes a stock. It is always measured “per something”: litres per minute, euros per week, pupils per day. Flows are the only way in or out. Here are stocks and flows from everyday life:

Feedback loops

Things get interesting when a stock influences its own flows. Follow the links and you go round in a circle: a feedback loop. There are only two kinds.

Most real systems have both kinds. Which one is stronger can change over time, and that is why things often grow fast at first and then level off. Move the sliders and watch a rumour spread through a school of 600 pupils:

Delays

Many effects arrive late. You turn the shower tap, but the hot water needs a few seconds to travel up the pipe. If you keep turning until it feels right, you have already turned too far. The result: too hot, then too cold, then too hot again.

Delays plus impatience cause swings everywhere: heating that overshoots, shops that order too much and then too little, pig prices that go up and down in cycles. The cure is usually the same: react more gently, or shorten the delay.

Surprising influences

Because everything is connected, a change in one place can show up somewhere you would never look. These effects are called side effects, but the system does not know which effects we wanted. Some famous examples, each a free model in the library:

• Wolves and rivers. When wolves came back to Yellowstone National Park, deer avoided the riverbanks, young trees grew back and the banks became more stable. Scientists still debate how strong the effect was, but the chain of links is a good example. Open the model
• More lanes, more traffic. A wider road makes driving quicker, so more people drive, until the road is full again. Traffic planners call it induced demand. Open the model
• The cobra effect. A reward for every dead snake made breeding snakes profitable. When the reward stopped, the snakes were released. Open the model
• The homework spiral. Staying up late to catch up leaves you tired and slower, so you fall further behind. Sleep influences school results. Open the model

Together is not the same as cause

In summer, ice cream sales go up and so does sunburn. If you only looked at those two numbers, you might think ice cream causes sunburn. Of course it does not: sunshine causes both. Two things that move together do not prove that one causes the other. There may be a hidden common cause.

Drawing a model helps here, because you have to draw every link you believe in. If there is no arrow from ice cream to sunburn in your drawing, no slider for ice cream will change the sunburn. Open the ice cream model and check.

Your first model in LeverScope

Let us build the piggy bank. It takes about five minutes.

1. Open LeverScope. On the left, click Stock and click on the canvas. Name it Savings and give it a starting value, for example 0.
2. Click Flow, then drag from empty space onto the stock. Name it Pocket_Money_Saved. Set it to 5 (euros per week).
3. Add a second flow from the stock out to empty space, called Treats. Set it to Savings*0.05: you spend 5% of your savings each week.
4. In Run settings, set the time unit to weeks and Stop to 104 (two years).
5. Press Play. Watch the savings grow and then level off. Can you explain why it stops growing?
6. Click Converter, add one called Pocket_Money with the value 5, and in its Simulation tab tick Show as a slider. Change the flow’s equation to Pocket_Money. Now you can drag the slider and see what changes.

The tool has its own handbook with 19 short chapters (button Learn at the top of LeverScope), from first steps to finding the best decisions and testing what could go wrong.

For teachers

Systems thinking fits into maths, geography, biology, economics and ethics lessons. All models in the library are free to use in class. No pupil account is needed to build and run models.

A 45-minute lesson that works:

1. Ask: “If I turn the bathtub tap down, does the water go down?” Let pupils vote. (5 min)
2. Run the bathtub on this page together and discuss the result. (10 min)
3. In pairs, pupils pick a stock from their life and name its inflows and outflows. (10 min)
4. Open the rumour model. Each pair predicts the shape of the curve on paper, then runs it. (15 min)
5. Share: where else have you seen an S-shape? (5 min)

Words to know