Mistake Proofing

Whenever lots of work is being checked for errors or reworked because of mistakes, it’s time to consider some mistake proofing. No one wants to waste his or her time checking for defects and fixing them. People want to spend their time on productive work. Making sure errors can’t happen in the first place eliminates that waste and lets people get back to doing work instead of fixing errors.

water cooler poka yoke

Note the extra button at the back of the red lever. People can’t accidentally burn themselves with hot water.

Mistake proofing, as a concept, isn’t unique to Lean, though the Japanese term for it—”poka yoke” (two syllables per word)—has to be the most fun way to express that concept in any language.

The literal meaning of poka yoke is “avoiding inadvertent errors.” Conceptually, it just means processes, devices, or systems designed so you can’t get it wrong by accident. Or as one of the King County Lean specialists likes to say, “no wrong moves.”

Note that mistake proofing isn’t quality checking or inspecting; it really means preventing errors from happening in the first place.

Real-world examples of poka yoke abound. For example, it is easy to get cold water out of most water coolers, but you have to press an extra button if you want hot water, so you can’t accidentally burn yourself.

FSU Team: Jean Prepotente, Brett Miller, and Mari ConradThe FSU Functional Analyst team (from right to left: Jean Prepotente, Brett Miller, and Mari Conrad) explains their work at the A3 Fair.

Here’s a poka yoke county employees came up with:

King County’s Financial Services Unit (FSU) acts as treasurer for 160 special districts (local governing bodies like water and school districts). In June 2014, mistakes appeared in two-thirds of the special district journal entry forms that FSU received from the districts! Each time FSU received a form with a mistake, it required a lot of extra work for them and the districts.

The sorts of mistakes that FSU was receiving were easy to make, especially as the people inputting the data work in other organizations. For example, account codes should have nine numbers, but some forms would have three digits, or an alphanumeric, or just plain language in the account line.

FSU employees can laugh at themselves now, but at the time, they would wonder, “Who doesn’t know a fund code is nine digits?” As if that sort of knowledge is innate, like breathing. (What would they say if they knew that one of our very own CIT members only recently learned that “task” on a standard resource use form was asking for a numeric code, not “meeting in Renton.”)

FSU Poka Yoke form

The newly poka yoke’d form. Users must enter five digits in the account fields and nine digits in the funds field. If they don’t, they get an error message.

No one makes these errors intentionally but it is easy to do…unless there’s a poke yoke.

So the FSU folks came up with a poka yoke. They created a form that won’t accept anything other than nine digits in the fund field, five digits in the account field, and so on. If special districts try to submit anything other than appropriate type of information in each field, they now get an error message, just like when you submit a credit card without an expiration date on a shopping website.

Of course, people could still put in the wrong five or nine digit code (an opportunity for more mistake proofing?), but FSU’s poka yoke prevents the true mistake that occurs when users just don’t know what kind of information is being requested. Since instituting this poka yoke, FSU has seen errors drop from more than 66% in June 2014 to less than 12% in September.

That improvement has saved oodles of time for the team. Each error wasted about five minutes of work time for Mari Conrad. When there were as many as 150 errors per month, that made for more than a full workday each month spent on correcting errors.

“We used everybody’s strength to address this problem,” says Jean Prepotente describing how she and Brett Miller helped Conrad to fix something that was wasteful and frustrating in her day. “That’s what makes us a great team.” The teamwork saved Conrad 10 hours or more per month that she can now spend helping the team solve other problems.

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