Mistake-proofing (poka-yoke)
Poka-yoke means “mistake-proofing” in Japanese. It is a device or design feature that prevents an error from occurring or immediately detects it if it does. Green Belt exams test the difference between prevention and detection poka-yokes, the levels of robustness, and common workplace examples.
Prevention vs. detection
| Type | What it does | Preferred? | Example |
|---|---|---|---|
| Prevention | Physically or mechanically stops the error from being made or from proceeding | Yes — stronger | A USB connector that can only insert one way; a jig that won’t clamp unless the part is correctly oriented |
| Detection | Detects the error after it has occurred and signals or stops the process | Second choice | A sensor that triggers an alarm when a lid is not seated; a vision system that rejects a misassembled unit |
Levels of poka-yoke (strongest to weakest)
- Elimination: Redesign so the error is physically impossible. The most robust level. Example: redesigning a two-piece assembly into one molded part so the assembly step (and its errors) no longer exists.
- Prevention: A physical feature stops the error or prevents the process from continuing after one. Example: a machine that locks if a safety guard is open.
- Detection: The error is detected and the product is stopped or rejected before reaching the next step. Example: a weight check that diverts underweight units.
- Warning: An alarm or indicator notifies the operator that an error has occurred or is about to. The weakest form — relies on human response. Example: a beep when a door is ajar.
Common mistake types and poka-yoke approaches
| Mistake type | Example | Poka-yoke approach |
|---|---|---|
| Wrong orientation | Part inserted upside-down | Asymmetric shape; locating pin that prevents wrong orientation |
| Missing step | Operator skips a process step | Checklists with mandatory electronic sign-off; sequence interlock that prevents step N+1 until step N is confirmed |
| Wrong part | Incorrect component used | Kanban kitting that delivers the right part only; barcode scan that must match before proceeding |
| Quantity error | Too few fasteners installed | Counted kitting trays; torque tools that count cycles before releasing |
| Measurement error | Incorrect dimension set on machine | Digital readout with set-point lock; go/no-go gauge |
Exam tip
When a question asks the Green Belt to “mistake-proof the process,” the correct answer is almost always a prevention or elimination approach, not additional inspection. Inspection is a detection control, which is better than nothing but inferior to preventing the error from occurring.
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