🛁Stocks and flows

A stock is anything that accumulates and can be counted at one moment: water in a bath, money in an account, tickets in a backlog, trust. A flow is a rate over time that fills or drains it. The whole idea is one sentence from chapter one of 📕 Thinking in Systems, in my paraphrase: a stock changes only through its flows. Nothing else touches it.

How it works

• Inflow greater than outflow: the level rises. Smaller: it falls. Equal: it holds, however fast both are running.
• A stock can only change as fast as its flows allow, so stocks are slow. That slowness is a buffer and a source of delay at once.
• People watch inflows. The outflow is the half that gets forgotten.

The bathtub. Tap, plug, water level. Turn the tap down and the bath is still full. I drew six of these and keep them in Bathtub diagram set.

Often confused with: a list. A list can be sorted. A stock can only be filled or drained. With one equation it becomes 📐 Little's law.

Taught to me by Meadows; my notes on the chapters are 📖 Meadows on stocks, flows and delays.

Where I have seen it

The backlog. In January ours was treated as a list, and so every planning meeting re-ranked it. Re-ranking is neither an inflow nor an outflow. The level did not move.

• Inflow: requests from Sales, Support, customers and me.
• Outflow: things finished, and things deleted. Deleting counts.
• Level: up, every quarter I have looked.

What I concluded from that is a different note, 💎 A backlog is a stock and only the outflow shrinks it, and in June it became 📝 Issue 42 - Your backlog is a stock, not a list.

Also seen in: the sabbatical fund (one inflow on the 27th, an outflow I flinch at), and the 214 open exceptions on a dispatcher's screen in Rotterdam, which were a stock being shown as a to-do list.