Stocks and Flows, explained.
Stocks and flows distinguish an accumulated quantity from the rates adding to or removing from it, explaining why a reduced inflow can still leave a level rising.
Why it happens
The stored amount changes by inflow minus actual outflow over time. Rates describe change; the stock retains previous accumulation. A tank with finite capacity can overflow, and an empty tank cannot supply its full drain capacity.
A stock is an accumulated quantity; flows add to or remove from it. Changing a rate is different from changing the level that has already accumulated.
Read the result
Run five minutes at a time and watch the stored amount. Compare the selected minute with the full curve. The drain control sets a maximum, while actual drainage is limited by the water available.
A worked example
A backlog keeps rising
A team has forty unfinished tasks, receives six per day and completes four.
Its backlog grows by two per day. Reducing arrivals to five slows the growth but does not reverse it.
To shrink the backlog under comparable conditions, completions must exceed arrivals. A smaller growth rate is not a smaller stock.
OPTIONAL DEEPER DETAILGo deeper: inside the model
Inside this model
A tank starts with 40 units and has capacity 100. Each of 20 one-minute steps adds the inflow, drains up to the available amount, then caps storage at 100. Excess is counted as overflow. Drain capacity is not actual outflow when the tank is empty.
Where this idea is useful
A practical use
A smaller backlog arrival rate does not shrink a queue unless departures exceed arrivals. Measure the existing backlog separately from the change per period.
A common misconception
“Reducing the inflow immediately reduces the accumulated level.”
The direction depends on net flow. The existing stock persists until removals exceed additions, apart from other mechanisms.
What this explanation leaves out
- Rates are fixed, time is discrete, and there is no feedback delay. The ordering of flow updates is explicit; real systems may have continuously changing rates and constraints.
Why can the drain capacity differ from actual outflow?
A drain capable of seven units cannot remove seven when fewer are available. Capacity and realized flow should be measured separately.
Are you trying to change the rate of accumulation or the amount already accumulated?
Associated thinkers
Further reading
Explore the original research or the teaching reference behind this experiment.