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 developed it at Toyota in the 1960s. 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.
| Approach | What it does | Illustrative example | Use it when |
|---|---|---|---|
| Prevention | The error physically can't happen | A fixture that only accepts the part the right way round | The error can be designed out of the part, tool or fixture |
| Detection | The error is signalled or the process stops as soon as it happens | A sensor that stops the press when a hole is missing | 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.
| Method | Catches | Typical devices | Example |
|---|---|---|---|
| Contact | A wrong physical attribute: shape, size, orientation, color | Guide pins, shaped fixtures, limit switches, color coding | An asymmetric guide pin for an engine mount |
| Fixed-value | A wrong number of parts or actions | Counters, trays with one pocket per part, fill checks | An eight-pocket fastener tray on an automotive sub-assembly |
| Motion-step | Steps done out of order or skipped | Interlocks, sequence-checking tools, checklists that unlock the next step | 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.
How to add poka-yoke to a process
- 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
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
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
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
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
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
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.
Go deeper
This page is the short version. These guides cover each part in full.
- Poka-Yoke: Meaning, 3 Types and How Mistake-Proofing WorksWhat the term means and how prevention and detection work, with the three types explained.
- 10 Poka-Yoke Examples from Real Manufacturing LinesTen examples from automotive, electronics, medical, pharma and aerospace lines, grouped by method.
- 3 Types of Poka-Yoke: Choosing the Right Method for ManufacturingDevice types and design principles for each method, and how to choose or combine them.
- Poka-Yoke: Error Proofing Methods in ManufacturingThe origins at Toyota, why discipline alone doesn't stop human error, and how to measure a device's effect.
- Designing Poka-Yoke Devices: Implementation GuideFrom root cause to prototype: the design process, validation testing and common design mistakes.
All our articles on this subject are in the quality management topic.
Free templates and tools
- Fishbone diagram template (6M)A printable six-M fishbone on one landscape sheet, plus the fishbone board in Lean Studio: problem statement at the head, a bone for each M and a table for the causes you will check.
- Poka-yoke worksheetEach step's possible errors, prevention or detection, the method, a device idea and a verified box.
- 5 Whys templateA problem statement, five why rows with evidence, the root cause and the countermeasure.
- 8D report templateD0 to D8 on three pages: team, is/is not, containment, occurrence and escape causes, actions and closure.
- First pass yield (FPY) calculatorFirst pass yield at each step and rolled throughput yield (RTY) across the whole process.
- Cost of poor quality (COPQ) calculatorTotal COPQ and cost of quality, and each as a share of revenue, from the four quality cost categories.
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