What is poka-yoke?
Poka-yoke means mistake-proofing. People make mistakes, however well trained and careful; poka-yoke accepts that and changes the process instead, so that the mistake cannot happen or is caught the instant it does. It is the answer to "operator error" in a root cause analysis: if a person could make the mistake, the process allowed it. Good poka-yoke devices are simple, cheap, built into the process and checked regularly.
The mistake-proofing control hierarchy
| Level | Control | Example |
|---|---|---|
| 1. Eliminate | Design the error out | Asymmetric part that only fits one way |
| 2. Prevent | The process cannot continue until it is right | Press will not cycle until a sensor confirms the O-ring |
| 3. Detect the error | Signal the error as it happens | Light and buzzer when a step is skipped |
| 4. Detect the defect | Catch the defect before the part leaves the station | Go/no-go gauge after machining |
| 5. Inspection or attention | Rely on people | Visual inspection, training, checklists |
The three poka-yoke methods
- Contact method: detects shape, size, colour or presence (guide pins, limit switches, vision sensors).
- Fixed-value method: checks that the right number of parts or actions is complete (counting nutrunner, kitting tray with one pocket per part).
- Motion-step method: checks the sequence (an interlock that requires step 3 before step 4, a scanner that refuses untested units).
Poka-yoke examples
The example worksheet covers a pump assembly line. The highest-priority mistake, a loose housing screw (severity 8 × occurrence 5), was checked by an operator with a torque wrench; a counting nutrunner now releases the fixture only after six good torques, moving it from "detect the error" to "prevent". A cover that could be fitted rotated is designed out with an asymmetric locating pin, the strongest level. A skipped function test, rare but severe, is prevented by a packing scanner that rejects units without a pass record. Each change also updates the FMEA and the standard work.
Mistakes found in a complaint? Use 5 Whys to find where the process allowed the error, then come back here to design the device.
How to use this Poka-Yoke tool
- List the mistakes. Walk the process and list each step where a mistake can happen, with its effect.
- Rate the risk. Rate severity and occurrence from 1 to 10. The ladder sorts mistakes by severity × occurrence.
- Record today's control. Choose how each mistake is controlled today, from elimination down to inspection or attention.
- Design the poka-yoke. Describe the device, its method (contact, fixed-value or motion-step) and the level of control it will give.
- Read the ladder. Green arrows show how far each device moves the control up the hierarchy.
- Track and export. Assign owners and status, then export the worksheet to PDF or Excel and update your FMEA.
Frequently asked questions
What is poka-yoke?
Poka-yoke (Japanese for "mistake-proofing") is any device or design that prevents a human error from happening, or detects it immediately so it cannot turn into a defect. The term was coined by Shigeo Shingo at Toyota. Everyday examples are a USB plug that only fits one way and a car that will not start unless it is in park.
What are the three poka-yoke methods?
Contact methods detect a physical attribute such as shape, size, colour or presence (locating pins, sensors, vision). Fixed-value methods check that a set number of actions or parts is complete (counting screws, kitting trays). Motion-step methods check that steps are done in the right order (interlocks, sequence checks).
What is the difference between prevention and detection poka-yoke?
A prevention (control) poka-yoke makes the mistake impossible or stops the process before it can continue, for example a fixture that only accepts the part the right way. A detection (warning) poka-yoke signals the mistake when it happens, with a light or alarm, and relies on someone to react. Prevention is stronger.
What is the hierarchy of mistake-proofing?
From strongest to weakest: eliminate the possibility of the error by design; prevent it (the process cannot be done wrong or cannot continue); detect the error as it happens; detect the resulting defect before it leaves the station; and finally rely on inspection, training and attention. Aim to move every high-risk mistake as far up the hierarchy as you can.
How does poka-yoke relate to FMEA?
A process FMEA lists failure modes and their causes; many causes are human errors. Poka-yoke devices are among the most effective actions to lower occurrence (by preventing the error) or improve detection. After implementing one, update the FMEA ratings and the control plan, and verify the device works at every shift start.
How do I prioritize which mistakes to error-proof?
Start with mistakes that have severe effects and happen often. This tool ranks them by severity × occurrence (1–10 each). Also consider mistakes that are hard for people to notice, such as missing small parts, wrong variants and skipped steps.