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Prevent Mistakes with Poka-Yoke

by Ron Kurtus (revised 14 November 2001)

It is cost effective for a company to employ means to prevent errors or mistakes that may cause defective parts to be produced. This also includes preventing the product to be incorrectly used.

One assembly-line method is to use a poka-yoke device in the process. Another approach is to make parts and equipment "idiot-proof" so that they may not be used incorrectly.

Questions you may have include:

This lesson will answer those questions.



Preventing mistakes

Instead of simply inspecting for mistakes and eliminating bad parts or using SPC to improve the processes, a different approach is to use devices or designs that will help to prevent mistakes in the first place. This approach of trying to make it difficult for the worker to make mistakes is credited to Mr. Shigeo Shingo, an industrial engineer at Toyota.

Poka-yoke device

Poka-yoke is a simple device or method to prevent mistakes at their source. (Poka-yoke is Japanese for "mistake proofing.") These devices are used either to prevent the special causes that result in defects or to inexpensively inspect each item that is produced to determine whether it is acceptable or defective.

Eliminate errors

The cause of many defects lie in worker errors. The defects are the results of neglecting those errors. Thus, if you eliminate worker errors, many defects will be eliminated.

Devices

Some examples of using poka-yoke devices are:

Such simple methods or devices anticipate potential sources of worker error. In such cases, they are often an effective alternative to demands for greater worker diligence and exhortations to "be more careful."

Helps before-the-fact inspection

Effective poka-yoke devices make before-the-fact inspection more effective by reducing the time and cost of inspection to near zero. Because inspections entail minimal cost, every item may be inspected. Provided that work-in-process inventories are low, quality feedback used to improve the process can be provided very rapidly.

Idiot-proof design

Another method to avoid mistakes is to design items so that they can only be assembled in a certain way. They call this an "idiot-proof" design because it does not take much intelligence to put the parts together.

Example

An example of an idiot-proof design is the three-prong electrical plug. There is only one way that you can plug it into the wall socket.

Plug can be put into socket only one way

Plug can be put into socket only one way

Surprising problems

Although an effort to idiot-proof products and assembly-line processes, there are sometimes people who attempt to do some amazing things. For example, there was a news item several years ago about a lawsuit by a woman who was injured by a soft drink bottle.

She tried to get a twist-off cap from the bottle by turning it the wrong way. When it wouldn't come off, she used a pair of pliers until she finally broke off the top of the bottle, injuring herself. An idiot-proof design might not work with such a person as a customer or on the assembly-line.

Complex processes

Of course idiot-proof design is much more difficult in complex processes. One example where such a design works in complex processes is the use of Installation Wizards for many software applications. They are almost idiot proof.

Can you think of any other high-tech examples?

Summary

A viewpoint of preventing mistakes before they occur is the best way to reduce failures and waste, resulting in lowered costs. This philosophy holds not only on the production line, but also in the office and for the work that managers do.

In some cases simple poka-yoke devices can help prevent mistakes.


Design your work and life for quality


Resources and references

Ron Kurtus' Credentials

The following resources provide information on this subject:

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Books

Top-rated books on Total Quality

Top-rated books on Business Quality


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