Poka-Yoke: How to Mistake-Proof a Process

Poka-yoke is Japanese for mistake-proofing. It means designing a process so a mistake cannot be made, or is spotted the moment it is made. The idea was developed by Shigeo Shingo at Toyota in the 1960s. His starting point was that people will always make occasional mistakes, so the process should catch them rather than relying on people never slipping.

You meet poka-yoke every day. A car will not let you take the key out unless it is in park. A USB-C plug fits either way up. A form will not submit without a valid postcode.

Three levels of mistake-proofing

From strongest to weakest:

1. Prevent. Make the mistake physically impossible. A jig that only accepts the part the right way round. A drop-down list instead of free typing.
2. Detect. Let the mistake happen but make it obvious at once, before it goes any further. A light that comes on when a step is missed. A spreadsheet cell that turns red when the total does not match.
3. Check. Add an inspection before the work reaches the customer. This is the weakest option because it relies on someone checking properly every time.

Always look for a preventive fix first. A checklist is better than nothing, but a process where the mistake cannot happen is better than a checklist.

Examples outside the factory

These are illustrations of the idea:

• An electrician's van: a printed kit list taped inside the door, checked before leaving the yard.
• An accounts office: invoices raised from the job sheet in the system, so the price cannot be typed wrongly.
• A kitchen: colour-coded chopping boards, so raw meat is never prepared on the board used for salad.
• An online shop: a barcode scan at packing that rejects the wrong item before the box is sealed.
• A dental practice: a booking system that will not offer a slot too short for the treatment chosen.

What does the mistake cost you?

A mistake that happens a dozen times a month can cost far more than people think, especially when some of them reach the customer. Enter one repeat mistake and the cost of a proposed fix.

In the example, each mistake takes 90 minutes to put right at £30 an hour, wastes £25 of materials, and one in four reaches the customer at a further cost of £150. That is £45 + £25 + £37.50 = £107.50 per mistake, or £15,480 a year at twelve a month. A £600 fix that prevents 80% of them pays back in under a month.

How to choose a fix

1. Find where the mistake is made, not where it is found. Inspection at the end shows where it was found.
2. Ask why it can be made at all. The 5 Whys helps here.
3. Look for the cheapest strong fix. Most good poka-yoke devices are simple: a guide, a template, a colour code, a required field.
4. Test it for a month and count the mistakes again.
5. Write it into the procedure so it survives staff changes.