Poka-yoke: a guide to mistake-proofing

Poka-yoke is Japanese for mistake-proofing: poka means an inadvertent mistake and yoke means to avoid. It means designing a process step so that an error cannot be made, or is noticed the moment it is made, so a defect never reaches the next step. It is also called error-proofing.

Contact methods catch a wrong physical attribute such as shape, size or orientation. Fixed-value methods catch a wrong quantity, such as a missing part. Motion-step methods catch steps done in the wrong order or skipped.

A prevention device makes the error physically impossible, for example a fixture that only accepts the part one way. A detection device lets the error happen but signals it or stops the process at once, for example a sensor that stops the machine when a part is missing. Prevention is preferred where it is practical.

Shigeo Shingo, an industrial engineer who taught at Toyota as an outside consultant from 1955, developed it in the early 1960s. In Zero Quality Control (1986) he describes a 1961 visit to Yamada Electric, where workers sometimes forgot one of two springs in a push-button switch; laying out both springs in a small dish before each assembly made a missed spring obvious. He first called such devices baka-yoke (fool-proofing) and renamed them poka-yoke around 1963. The method became a standard tool of the Toyota Production System. He started from the view that human error is inevitable, so the process, not the worker, has to stop it.

An asymmetric guide pin on a fixture so an engine mount can only go in one way (contact method), a tray with one pocket per fastener and a sensor that stops the unit moving on until every pocket is empty (fixed-value method), and a torque tool that only registers the bolt lit up on the fixture, so the sequence can't be skipped (motion-step method). Our examples post describes ten, from automotive, electronics, medical, pharma and aerospace lines.

Yes. The device has to stop the error that causes the defect, so find that error first with a tool such as the 5 Whys or a fishbone diagram. A device aimed at the defect leaves the error in place.

Poka-yoke is any device or method that prevents an error from being made, or makes it obvious at once so it cannot become a defect. Examples are a fixture that only accepts a part the right way round, a sensor that stops a machine when a step is skipped, and a connector that cannot be mated incorrectly. It builds quality in at the source instead of relying on inspection.

Any device that stops or exposes a mistake. A guide pin that lets a part fit only one way, a connector with a shape that cannot be plugged in backwards, a tray with one pocket per fastener plus a sensor that holds the unit until every pocket is empty, and a limit switch that stops the machine when a part is missing are all poka-yoke. The first two prevent the error and the last two detect it.

People make mistakes however careful they are, and inspection finds a defect only after it has been made. Poka-yoke stops the error at the step where it happens, so the defect never reaches the next step. That means less rework and scrap and less dependence on people remembering. Shigeo Shingo started from the view that human error is inevitable, so the process, not the worker, has to stop it.