Poka-Yoke or “Mistake Proofing”

Poka-yoke (ポカヨケ) is a Japanese term that means “mistake-proofing” or “inadvertent error prevention“. A poka-yoke (mistake proofing) is any mechanism in a process that helps an equipment operator avoid (yokeru) mistakes (poka) defects by preventing, correcting, or drawing attention to human errors as they occur. The concept was formalized, and the term adopted, by Shigeo Shingo as part of the Toyota Production System. Fool-proofing, error-proofing; A control method that makes it very unlikely or impossible for an error to occur.

Poka-yoke (mistake proofing) can be implemented at any step of a manufacturing process where something can go wrong or an error can be made. For example, a fixture that holds pieces for processing might be modified to only allow pieces to be held in the correct orientation, or a digital counter might track the number of spot welds on each piece to ensure that the worker executes the correct number of welds.

Shigeo Shingo recognized three types of poka-yoke (mistake proofing) for detecting and preventing errors in a mass production system:

  1. The contact method identifies product defects by testing the product’s shape, size, color, or other physical attributes.
  2. The fixed-value (or constant number) method alerts the operator if a certain number of movements are not made.
  3. The motion-step (or sequence) method determines whether the prescribed steps of the process have been followed.

Either the operator is alerted when a mistake is about to be made, or the poka-yoke device actually prevents the mistake from being made. In Shingo’s lexicon, the former implementation would be called a warning poka-yoke, while the latter would be referred to as a control poka-yoke.

Shingo argued that errors are inevitable in any manufacturing process, but that if appropriate poka-yokes are implemented, then mistakes can be caught quickly and prevented from resulting in defects. By eliminating defects at the source, the cost of mistakes within a company is reduced.

References

Wikipedia. Poka-yoke. https://en.wikipedia.org/wiki/Poka-yoke