Key takeaways
  • Poka-yoke means mistake-proofing: changing the work so the error is impossible or immediately obvious, rather than training people to avoid it.
  • Prevention beats detection. A fixture that only accepts the part one way is better than a sensor that alarms afterwards.
  • If your corrective action is "remind the operator", you have not mistake-proofed anything.
The test of a real poka-yoke
Could a new starter make this mistake on their first day?

If the answer is yes, and the only thing preventing it is attention, you have a procedure rather than a poka-yoke. The point of the technique is to remove reliance on memory and care entirely.

The three types

  • Contact. Physical shape, size or fit prevents the wrong action. An asymmetric fixture that only accepts the part in the correct orientation.
  • Fixed value. A count or quantity check. A tray holding exactly the number of fasteners required, so a leftover means a missed step.
  • Motion step. Sequence enforcement. The next step cannot begin until the previous one is confirmed.

Prevention versus detection

A prevention device makes the error impossible. A detection device makes it immediately visible. Prevention is always preferable, but detection at the point of the error is a strong second, and both beat finding the defect at final inspection.

The value falls sharply with distance from the error. A sensor that catches a missing component at the next station is worth far more than the same catch at despatch.

Choosing one for a recurring defect

  • Identify precisely where the error is made, not where it is found.
  • Ask whether shape, count or sequence could make it impossible.
  • If prevention is not feasible, detect at the moment it happens.
  • Prefer the cheap and physical over the clever and electronic. Jigs and trays outlast software.
  • Test with someone unfamiliar with the job. If they cannot make the mistake, it works.

Worked examples

  • A connector keyed so it cannot be inserted the wrong way round.
  • A kit tray with a recess per component, where a filled recess at the end means a part was not fitted.
  • A torque wrench that will not release the next step until the correct torque is reached.
  • A scale under an assembly station that flags an out-of-tolerance total weight, catching a missing or duplicated part.

Where checklists fit

A checklist is a weak poka-yoke: it relies on the person reading and honestly answering it. It becomes considerably stronger when a step cannot be skipped, when a specific answer forces a follow-up action, and when the record shows what was missed. That is the difference between a printed list and an enforced sequence.