Poka-yoke (Wikipedia)
Wikipedia’s article on poka-yoke (“mistake-proofing”), Shigeo Shingo’s Toyota Production System term for redesigning a task so a mistake can’t physically happen, rather than trusting a person to avoid it through care.
License: CC BY-SA 4.0 — https://creativecommons.org/licenses/by-sa/4.0/
Key points
- Also called a forcing function or behavior-shaping constraint. Shingo distinguished a mistake (the human act) from a defect (a mistake that reaches the customer) — poka-yoke’s job is to stop the first from becoming the second.
- Detection methods: the contact method (tests a physical attribute — shape, size, color — against what’s expected), the fixed-value method (alerts if a required number of movements/parts wasn’t completed), and the motion-step method (verifies the prescribed sequence of steps was actually followed).
- Implementation types: a warning poka-yoke alerts the operator before the mistake completes, giving a chance to stop; a control poka-yoke physically prevents the mistake from happening at all — control is the stronger of the two.
- Worked example: assembly workers kept forgetting to insert a required spring under a switch button. Shingo split the task in two — prepare both required springs in a placeholder first, then insert them into the switches — so a spring left in the placeholder immediately signals a skipped step, catchable “effortlessly” rather than downstream.
- Everyday examples: a keyed connector (HDMI, USB-C) that only fits one way; a manual-transmission car that won’t start without the clutch depressed; an automatic-transmission car that requires Park/Neutral or the brake pedal; a microwave that can’t run with its door open and stops the instant the door opens mid-cycle.