🚿 Controls
1 Set the input rate (new orders released)
2 The output (capacity) is fixed at bottom
3 If input > output, WIP rises. Watch lead time explode!
💼 Interview Tip
The textbook uses a funnel/bathtub analogy for I/O control: new orders pour in through the input valve, WIP is the water level, and capacity is the drain. The key rule: planned input should never exceed planned output. When it does, backlogs build, lead times grow, and congestion makes everything worse. Mention Little's Law: Lead Time = WIP / Throughput to connect the concepts.
Instructor Notes
- Start balanced (input = 12, output = 14). WIP stays low and stable.
- Then overload: set input to 20. Watch the bathtub fill up and lead time climb tick by tick. This is visceral.
- Key debrief: the textbook says "when input exceeds output, backlogs build, congestion occurs, and flow to downstream workcenters becomes sporadic."
- Connect to TOC: the bottleneck sets the output rate. The rope should limit input to match it.
📊 The Bathtub
Input: 12/hr
0
Output: 14/hr
Throughput
-
Avg WIP
-
Avg Lead Time
-
Utilization
-
Cumulative Input
Cumulative Output
WIP Level
Adjust the input rate and watch what happens to WIP and lead time.