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Thinking in Diagrams

Chapter 6: What Accumulates

Drafted for Aryeh's edit.

For chapter 6: What Accumulates

Timing

Section titled “Timing”
  • Reading the chapter: about 11 minutes, its 1,662 words of prose at 150 words a minute. Not counted in those minutes: 5 figures, 1 pair, 2 understanding checks, the reading exercise and the exercises.
  • A session: 50 minutes, the exercises set for it. (estimate)
  • Afterwards: 55 minutes, the exercises set for then. (estimate)

Common misreadings

Section titled “Common misreadings”
  • A rate drawn as a stock. In the bad diagram, the tap and the drain are stocks counted in liters per minute on either side of the water in the tub; the lab’s check reports filling and draining as transfers between stocks in different units. In the understanding check about a phone, charging is the choice that reads a flow as a stock.
  • A stock’s behavior read from its inflow alone. Cronin, Gonzalez and Sterman (2009) found highly educated adults often unable to infer how a simple stock behaves, assuming its behavior matches the pattern of its flows: the correlation heuristic. In the understanding check about Dana’s inbox, They fall, because fewer emails arrive is this reading; arrivals still exceed what she clears.
  • A stock’s path drawn that its flows can’t produce. Booth Sweeney and Sterman (2000) found students at an elite business school, new to system dynamics, understood stocks and flows poorly. A stock’s slope is its net flow: in SF 02, burning outpaces gathering by a steady seven kilograms an hour, so the pile falls in a straight line.
  • A flow to a cloud left out of a stock equation. In the reading’s third question, change in groundwater = groundwater recharge − groundwater discharge to ocean leaves out agricultural water use, which drains groundwater to a cloud.
  • A flow counted in its stock’s units. In the reading’s second question, cubic kilometers is this choice for ocean evaporation; SF 01’s self-check counts the words Lena knows in words and the flow in words per week.

Exercises to set

Section titled “Exercises to set”

The session goes from one stock and its inflow, through two flows and a stock equation, to a published diagram of Earth’s water redrawn as stocks and flows; a delay, a chain of stocks and the case come afterwards.

For a session

Afterwards

Sources

Section titled “Sources”