Skip to content

Method guide · Quality management

Poka-yoke: a guide to mistake-proofing

Short answer

Poka-yoke is Japanese for mistake-proofing: designing a step so an error can't be made, or is caught the moment it is made, instead of relying on people to be careful. Shigeo Shingo, a consultant who taught at Toyota, developed it in the early 1960s, starting with a switch assembly at Yamada Electric in 1961, and it became a standard tool of the Toyota Production System. There are two approaches, prevention and detection, and three methods: contact (a wrong shape, size or orientation), fixed-value (a wrong count) and motion-step (a wrong or skipped sequence).

Prevention or detection

A poka-yoke either makes the error impossible or makes it obvious straight away, before the part moves on. Make it impossible if the part, tool or fixture allows it.

  • Prevention

    What it does
    The error physically can't happen
    Illustrative example
    A fixture that only accepts the part the right way round
    Use it when
    The error can be designed out of the part, tool or fixture

  • Detection

    What it does
    The error is signalled or the process stops as soon as it happens
    Illustrative example
    A sensor that stops the press when a hole is missing
    Use it when
    Prevention isn't practical, or as a second layer behind it

Our guide to poka-yoke error-proofing methods goes into both approaches and how to decide between them.

The three methods

Which method to use depends on the kind of error, so work out what goes wrong before you pick one.

  • Contact

    Catches
    A wrong physical attribute: shape, size, orientation, color
    Typical devices
    Guide pins, shaped fixtures, limit switches, color coding
    Example
    An asymmetric guide pin for an engine mount

  • Fixed-value

    Catches
    A wrong number of parts or actions
    Typical devices
    Counters, trays with one pocket per part, fill checks
    Example
    An eight-pocket fastener tray on an automotive sub-assembly

  • Motion-step

    Catches
    Steps done out of order or skipped
    Typical devices
    Interlocks, sequence-checking tools, checklists that unlock the next step
    Example
    A torque tool that follows the fixture sequence in aerospace assembly

The examples come from our 10 poka-yoke examples from real manufacturing lines. How to choose between the methods, and when to combine them, is in 3 types of poka-yoke. For 28 shorter ones from daily life and the shop floor, each with a picture, see the poka-yoke examples gallery.

How to add poka-yoke to a process

  1. 1

    Find where errors happen

    Start from defect records, rework and customer complaints. First pass yield by step shows where the process loses good parts.

  2. 2

    Find the root cause first

    Use the 5 Whys or a fishbone diagram to find the error behind the defect. A device aimed at the symptom leaves the error in place.

  3. 3

    Write down the error

    What goes wrong, at which step, and how you would know. That description is the specification for the device. The poka-yoke worksheet has a row for each one.

  4. 4

    Prefer prevention, and match the method

    Design the error out if you can. Then pick contact, fixed-value or motion-step to fit the kind of error.

  5. 5

    Start simple and build it with operators

    Try a pin or a shaped tray before a sensor. Prototype at the workstation with the people who do the job, since they know the ways around it.

  6. 6

    Test it by making the mistake

    Make the error on purpose, try parts at the edges of tolerance, and decide what happens if the device itself fails.

  7. 7

    Update standard work and keep checking

    Add the device to the procedure and to routine checks, then watch the defect rate to confirm it worked.

The poka-yoke device design guide covers each step, the validation tests and the most common design mistakes.

Where poka-yoke fits

  • Quality at the source: poka-yoke is one of the main ways to build quality into each step instead of inspecting at the end.
  • Jidoka: a detection device that stops the process on an abnormality is jidoka at the scale of one station.
  • FMEA: ranks failure modes, which tells you where mistake-proofing pays off first.
  • 8D and corrective action: a poka-yoke can be the permanent fix that stops a customer complaint from coming back.
  • First pass yield and the cost of poor quality: the measures that show whether the device paid for itself.
  • Zero Quality Control (Shingo, Productivity Press, 1986): the primary source, where Shingo sets out source inspection and the poka-yoke methods and tells the Yamada Electric switch story.

Go deeper

This page is the short version. These guides cover each part in full.

All our articles on this subject are in the quality management topic.

Free templates and tools

Key terms

Lean glossary

Doing this in LeanSuite

See it on your own lines

Bring the problem you are working on to a 45-minute demo. Or look around with sample data first, or pilot LeanSuite on your own lines for 90 days, with the setup done by our team.

Book a 45-minute demoClick through the interactive demoStart a 90-day pilot

FAQ

Questions about poka-yoke

  1. 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.

  2. 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.

  3. 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.

  4. 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.

  5. 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.

  6. 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.

More method guides

Or browse the topics, the full library and the lean glossary.

Rather see it on your own problem?

Bring the problem you are stuck on, and we will show you on a call how LeanSuite handles it.

Schedule Demo

Pass it on

We want LeanSuite to be the best place on the internet for lean help. If this was useful, send it to someone on your team or in your network who needs it.