Systems Theory

Stocks & Flows: Why Change Happens Slowly

Why is it impossible to lose weight quickly and keep it off? Why do companies with huge profits suddenly go bankrupt? Why do glaciers keep melting for decades after temperatures stabilize? The answer is one: stocks. Accumulators are a system's memory, its inertia.

  • **Bathtub**: The water level (stock) depends on the tap (inflow) and the drain (outflow)
  • **Population**: The number of people changes through births and deaths - this takes generations
  • **CO₂**: Emissions minus absorption. Even at zero emissions, CO₂ stays for hundreds of years
  • **Reputation**: Trust takes years to build but can be destroyed in a day

What is a Stock?

**Puzzle:** If every power plant in the world switched to solar panels tomorrow, when would global warming stop? In a year? The answer: **at least 30-50 years**. And it's not about technology - it's about the structure of the system.

**Stock (accumulator)** - a quantity that accumulates or depletes over time. Stocks are measured in units, not units per time.

**Examples of stocks:**

  • Water in a bathtub (liters)
  • Money in a bank account (dollars)
  • Population of a city (people)
  • CO₂ in the atmosphere (gigatons)
  • Reputation (trust)

**Intuition:** A stock is a 'snapshot' at a point in time. If it can be photographed - it's a stock.

Which of these is a stock?

What is a Flow?

**Flow** - the rate of change of a stock. Flows are measured in units PER TIME.

**Examples of flows:**

FlowUnits
Water from a tapliters/minute
Salarydollars/month
Birth ratepeople/year
CO₂ emissionsgigatons/year
Customer churncustomers/month

**There are two types of flows:**

  • **Inflow** - increases the stock (tap into a bathtub)
  • **Outflow** - decreases the stock (drain from a bathtub)

**Intuition:** A flow is a 'rate'. Time is required to measure it.

A company acquires 100 new customers per month. This is...

The Fundamental Equation

Here is the one formula worth knowing:

**Stock tomorrow = Stock today + Inflow - Outflow**

A stock can only grow if inflow > outflow. It can only fall if outflow > inflow. Sounds simple, but this is constantly overlooked!

**Equilibrium** is reached when inflow = outflow. At that point, the stock doesn't change.

A 100-liter bathtub currently holds 50 L. Tap: 10 L/min, drain: 5 L/min. In how many minutes will it overflow?

Why Stocks Create Inertia

Stocks are **buffers** in a system. They smooth out fluctuations and create delays.

**Three important implications:**

ImplicationExample
Slow responseEven at zero emissions, CO₂ stays in the atmosphere for hundreds of years
Delayed consequencesThe problem only becomes visible when the stock reaches a critical level
Resilience to shocksSavings protect against job loss

**Inertia of stocks:** 200 years of accumulated CO₂ won't disappear in 10 years. Half of it will remain after 100 years. Systems have MEMORY - and that memory is stored in stocks.

If flow = 0, the stock will also become zero

If flow = 0, the stock stays constant. For a stock to decrease, there must be an outflow.

If studying stops (flow = 0), knowledge doesn't disappear instantly. But without review (forgetting = outflow), it will slowly decrease.

Why would global warming continue even with zero emissions?

Key Ideas

  • **Stock** - an accumulator (water, money, CO₂). Measured in units
  • **Flow** - the rate of change of a stock. Measured in units/time
  • **Equation**: Stock(t) = Stock(0) + Inflows - Outflows
  • **Inertia**: even at zero inflow, a stock doesn't vanish instantly
  • **Equilibrium**: a stock stabilizes when inflow = outflow

What's Next?

Stocks & Flows are the language for describing system dynamics.

  • System Archetypes — Common combinations of stocks, flows, and feedback loops
  • Leverage Points — Where to intervene in a stock-flow system for maximum effect
  • Delays — Delays between flow and stock change

Вопросы для размышления

  • What 'accumulators' matter most in professional development? (knowledge, skills, reputation, network)
  • What flows are filling and depleting them?
  • Where can inflow be increased or outflow reduced in a concrete system?

Связанные уроки

  • st-01-feedback-loops — Loops govern the flows that fill and deplete stocks
  • st-14-delays — Delays between flow and stock change are the source of inertia
  • st-15-boundaries — System boundary defines which stocks to include in a model
  • st-03-archetypes — Archetypes describe recurring stock-flow-loop patterns
  • prob-04-bayes — Bayesian prior/posterior updates like a knowledge stock via evidence flow
  • de-01
Stocks & Flows: Why Change Happens Slowly

0

1

Sign In