How to use it
- 1. Read the mapTwo stocks (supply line, inventory), three flows. Valve thickness animates with the live flow rate.
- 2. Hit it with a stepDemand jumps at week 10. Watch inventory dip and orders react.
- 3. Turn the supply-line dialAt 0% you ignore what you already ordered - the classic mistake. Watch the oscillation.
- 4. Read the amplificationPeak order rise ÷ demand step = how much the policy amplifies. This is the bullwhip, born.
Stock & flow model
Ordering policy
Orders = expected demand + inventory gap ÷ adjustment time + supply-line gap × the dial above.
Physics & shock
The lesson
Sterman’s beer-game result: people ignore goods already on order. Ordering into a pipeline you can’t see is how a 30% demand step becomes an order spike of almost 90%.
Swipe the diagram sideways to see the whole loop.
A live supply-chain twin: supplier releases enter the inbound pipeline, arrive at the factory/DC stock, and leave as customer shipments. Vehicle speed, stock levels, queues and the reverse order signal all respond to the same feedback equations.
Inventory vs its target - oscillation means the loop is fighting itself.
Orders vs demand - the gap between the red peak and the grey step is amplification.
Try these experiments
- Set supply-line weight to 0%, delay 4 weeks: a +30% step produces a big overshoot and rings for months.
- Slide it to 100%: same shock, orders rise once and settle. Nothing else changed.
- Now stretch the delivery delay to 8 weeks at 0%: longer pipelines make ignorance costlier - the oscillation grows.
About this tool
The concept
System dynamics models a chain as stocks (accumulations) and flows (rates) tied together by feedback loops and delays - behaviour emerges from structure, not from bad luck.
How it works
Euler integration, dt = 0.25 week. Orders enter a pipeline stock and arrive after the delivery delay; the policy closes the inventory gap and, optionally, the supply-line gap.
Winning strategy
Count what you’ve already ordered. Full supply-line adjustment makes the loop critically damped - the entire bullwhip cure in one parameter.