Meadows on stocks, flows and delays
📚SOURCE
💎PERMANENT NOTES
🧵SYNTHESIS
Paraphrase, not quotation: chapters 1 and 2 of 📕 Thinking in Systems.
A stock is whatever has piled up: water in a bath, money in an account, tickets in a queue. It can only change through its flows, what comes in and what goes out, and it changes at the speed of those flows and no faster. That is why stocks are slow, and why they act as the memory of a system. A bath does not empty because you have decided it should.
The second chapter adds delay. When the person steering a stock sees its level late, and their correction also arrives late, the level overshoots and swings. Her car dealer orders against last month's sales and ends up with a full lot and then an empty one. The uncomfortable part: reacting faster made the swings worse in her model, and waiting longer calmed them.
For me this is the backlog. Sorting it is not draining it.
✨KEY IDEAS
A stock changes only through its flows, however urgently anyone wants it to change.
A stock can be raised by slowing the outflow as well as by increasing the inflow, and the two feel very different to the people involved.
Stocks let inflow and outflow run at different rates for a while, which is what makes them buffers.
In the dealership model, shortening the reaction time made the oscillation larger.
Which loop dominates can change over time, so the same structure behaves differently in different years.
📝READING NOTES
What she says, as I would explain it to Sam
Chapter 1 builds everything from a bathtub. Tap, drain, water level. The level is the stock, the tap and drain are flows, and the only way to move the level is through one of them. She then makes three points that I had never put into words:
- Stocks change slowly, even when flows change at once. Turn the tap off and the bath is still full.
- A stock is a buffer. It lets the inflow and the outflow disagree for a while, which is why warehouses, bank balances and backlogs exist at all.
- We watch inflows more than outflows. A level rises just as surely when less drains away.
Then feedback. A balancing loop pushes a stock toward a target (a thermostat, a coffee cooling to room temperature). A reinforcing loop feeds on itself (interest, a population, a rumour). Most of the interesting behaviour comes from the two acting on the same stock.
Chapter 2: the zoo, and the one animal I recognised
Her dealership has a stock of cars, customers taking them away and a manager ordering more. Add three ordinary delays: she averages sales before believing a change, she does not order the whole shortfall at once, and the factory takes time to deliver. The inventory starts to oscillate, and nobody in the story has done anything foolish.
She shows that a quicker reaction amplifies the swing and a slower one damps it. I believe the model. I do not believe it is a licence to wait: in her example the delivery delay is fixed. When a delay can be shortened (a test that reports in an hour and not a week), shortening it is better than out-waiting it. She would probably agree. The chapter does not say so.
Where it lands for me
- The Exceptions Inbox backlog is a bath with a wide tap. Every prioritisation meeting rearranges the water. See 🧩 Stocks and flows for the diagram.
- My own reading list, same shape. Fifty books planned for 2023 was an inflow number.
- Training is a delayed stock too: Bram's programme pays out weeks after the work.
What came out of it
- 💎 A backlog is a stock and only the outflow shrinks it
- 💎 Delays make a sensible reaction look like the wrong one
- One line in the ideas folder the same Sunday: draw the backlog as a bathtub. It turned into an issue in June.
More on the author in 🏛️ Donella Meadows. The second half of the book is in 📖 Meadows on why systems surprise us.
Came Back To It
- 19 June 2025: Issue 42 went out this morning. The whole argument is this note with the bathtub drawn properly.
- 9 February 2025: Reread chapter 2 after the first Goldratt note. The dealership and the factory are the same story told by two temperaments.
- 19 January 2025: Added the pointer to the second note.
Reading record
- Section read: Chapters 1 and 2
🗣️IN SHORT
- In : Thinking in Systems, chapters 1 and 2
- Meadows argues that : the behaviour of a system comes from its stocks, its flows and the delays in its feedback, not from the events on its surface
- by stating that : a stock moves only through its flows and never faster than they allow, and that a delayed correction produces overshoot and oscillation even when every decision in the loop is reasonable