Systems & feedbackField guide · Systems & feedback · 4 of 6
What would still be there if everything stopped?
Over variables and effects, a stock & flow diagram shows stocks, the flows that fill and drain them, clouds, and signals carrying readings to flows and to other variables.
A flow’s double line moves material from one stock or cloud to another. A signal, a single thin line, is an effect carrying a reading to a flow’s valve or an auxiliary, which it changes; no signal ends at a stock.
Filling runs from a cloud into water in tub and draining out to another; signals from tap setting and drain opening set the flows, and the water’s + signal to draining makes deeper water drain faster (chapter 6’s bathtub).
Draw one when something builds up or runs down, when cutting what comes in hasn’t stopped a total rising, or when something leaving one place has to turn up in another (chapter 6’s signals).
When nothing accumulates, a sentence does, or a causal loop diagram for effects running in a circle; how much there is now is a list of amounts. The diagram holds no numbers (chapter 6, what stays informal).
A causal loop diagram is redrawn by hand as a stock & flow diagram, what accumulates becoming stocks, filled and drained by flows that signals set.
Where the controlled variable accumulates, it is drawn on a feedback control diagram with a stock & flow diagram’s stock, flows and signals, the roles written above them.
Chapter 6, What Accumulates: stocks, flows, clouds, units, transfers, auxiliaries, signals, the stock equation, delays on flows and behavior-over-time graphs; exercises SF 01–05.
Chapter 7, Stocks Inside Feedback: loops through stocks, conservation, aging chains, material and signal delays, and capacity limits; exercises SF 06–10.
Donella H. Meadows, Thinking in Systems: A Primer (Chelsea Green, 2008), chapter 1, The Basics, on stocks, flows, clouds as sources and sinks, and the bathtub.
John D. Sterman, Business Dynamics: Systems Thinking and Modeling for a Complex World (Irwin/McGraw-Hill, 2000), chapter 4, Structure and Behavior of Dynamic Systems, on the shapes behavior over time takes; chapter 6, Stocks and Flows, on the notation and on telling a stock from a flow by freezing a system in time; chapter 7, Dynamics of Stocks and Flows, on reading a stock’s path from its net flow; chapter 11, Delays, on delays in material and in information; and chapter 12, Co-Flows and Aging Chains.