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Poka-Yoke: Mistake-Proofing

Poka-yoke (ポカヨケ), coined by Shigeo Shingo, means "mistake-proofing." Instead of relying on attention or inspection, poka-yoke devices make errors impossible or immediately detectable.

Two types of poka-yoke

Type	What it does	Example
Prevention	Makes the error impossible to commit	Asymmetric USB-A connector — can only be inserted one way
Detection	Immediately signals an error if it occurs	Seat-belt alarm sounds if ignition is on and belt is unfastened

Three detection methods

1. Contact method: a pin, sensor, or template detects incorrect shape or position
2. Fixed-value method: counts parts, fasteners, or keystrokes required
3. Motion-step method: checks that each step in a sequence has been completed

Poka-yoke at different stages

• Source inspection: check at the operation that creates the defect — before passing it on
• Self-check: operator checks their own work immediately after
• Successive check: next operator checks the previous operation's output

CQE exam tips

• Poka-yoke is a tool of mistake-proofing, not just error detection — prevention-type is the ideal.
• 100% inspection via poka-yoke at the source is more effective than statistical sampling downstream.
• Shingo's concept connects to the PDCA cycle: poka-yoke closes the Act loop permanently.

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