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Error-Proofing

Definition:

Error-proofing refers to the implementation of fail-safe mechanisms to prevent a process from producing defects. This activity is also know by the Japanese term poka-yoke, from poka (inadvertent errors) and yokeru (to avoid) - pronounced POH-kuh YOH-kay.

Although this common-sense concept has been around for a long time, it was more fully developed and popularized by Shigeo Shingo in Japan.

The philosophy behind error proofing is that it is not acceptable to make even a very small number of defects, and the only way to achieve this goal is to prevent them from happening in the first place.

In essence, error-proofing becomes a method 100% inspection at the source rather than down the line, after additional value has been added (wasted). Achieving extremely high levels of process capability requires this type of focus on prevention rather than detection.

You may not realize it, but you probably encounter many examples of error-proofing in your every-day life, as outlined below:

Examples From Every-Day:

  • The brake-shift interlock device on your automatic transmission vehicle prevents you from starting the engine unless the brake is depressed and the shift selector is in park or neutral.
  • A tennis ball hung from a screen in your garage can make parking easier, and avoid crashing into items stored at the front of the garage.
  • When you close a computer file, the operating system may ask you if you want to save your work first to prevent losing it inadvertently.
  • Childproof caps on medicine bottles keep children from taking medication that could be harmful.
  • Soft barrier hanging in front of the door opening at a drive-thru that hits the top of your vehicle to prevent you (hopefully) from crashing into the overhanging wall.
  • Anti-siphon valve on your outside garden hose outlet that prevents contaminated water from flowing backward into the water supply.
  • Speed-dial on your phone that improves accuracy as well as speed - if you hit the right speed dial button!
  • The engine control module on your vehicle that controls top-end speed so you don't over-drive that speed rating of your tires.
  • A patient about to undergo surgery for a problem on his right arm, concerned about reports of surgery accidents, wrote "Wrong Arm" with a magic marker on his left arm.
  • The spell-check feature on a word-processing program.

Examples From Industry:

  • A sensor connected to the electric nut-runner in an automobile assembly plant triggers a horn if a suspension assembly is moved to the next operation before a bolt is properly tightened.
  • Color-coding of components that otherwise look similar. Bins of parts that are electronically connected to the bill of materials through a bar code scanner, so that only the proper bin door opens for a given product.
  • Bar-code scanning at the check-out lane to eliminate errors and increase speed
  • Computer controlled inspection of a vehicle's electrical system that senses any disconnected modules and electronically "locks out" the shipping documents so the vehicle must be properly repaired and verified by computer retest before it can be shipped to a customer.
  • Fuse box lock-out procedures so that power cannot be turned on before an electrical repair is complete and all personnel are out of harm's way.
  • Guide pins on emblems on the back of a car to guarantee proper alignment.

Categories:

You can see from the examples above that there are three basic categories of error-proofing:

  • At the simplest level are WARNINGS that provide quick feedback of a potential problem, like a smoke-alarm or the warning lights on your car instrument panel.
  • Automatic SHUTDOWN controls force the problem to be corrected before resuming production, such as the vehicle electrical test mentioned above.
  • AUTOCORRECTION controls provide an integrated test-feedback-repair loop, such as the spell-checker in a word processing program.

Application:

Error-proofing is an excellent activity to involve the workforce in continuous improvement. Many poka-yoke ideas relieve stress from operators by eliminating the need to concentrate on mundane activities and by providing more capable tools to get the job done right consistently.

And remember, operations that require more finesse, adjustment, judgment are not just a quality risk, they also take more time, and are therefore more wasteful than they could be in terms of productivity and quality.

The first step in applying error-proofing principles is to identify all of the possible errors that can be generated by an operation. Using a Failure Mode and Effects Analysis (FMEA) is a great way to get started, because the FMEA asks many of the same questions, and provides a prioritization of the potential errors. You may also find that a Cause and Effect Diagram (Fishbone Chart) is a useful tool to organize your efforts.

After identifying the potential errors, involve the workforce in Brainstorming to generate ideas to provide Warning, Shutdown, or Autocorrection to the process.

Summary

Error proofing, or Poke Yoke, can greatly decrease defects in processes and increase speed of processes. By decreasing variation and designing a system where the right way is the only way to produce a product or service, error proofing, enables focus on the other variable factors in the process. The more error proofing built into processes, the less the defects associated with errors will be generated.


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