The 3 Levels of Lean Mistake-Proofing (Poka-Yoke): How to Prevent Defects and Boost Quality (Boston Edition)

Why Mistake-Proofing (Poka-Yoke) Is Essential in Lean manufacturing

In Lean manufacturing and continuous improvement, mistake-proofing—also known by its Japanese term Poka-Yoke—is a crucial method for ensuring quality at the source. By proactively preventing human error, organizations can significantly reduce waste, lower costs, and enhance customer satisfaction.

Not all mistake-proofing strategies are equal. There are three recognized levels of Lean mistake-proofing, each with a different impact on error reduction and process control. Let’s explore these levels and how you can apply them to optimize your operations. Then (of course!), we’ve got a wicked good example!

Level 3: Error Prevention (aka Cannot Produce)

At this level, the process and/or parts are designed so that errors are physically impossible to make. It’s the most effective form of mistake-proofing because it removes human variability from the equation.

   Examples of Level 3 mistake-proofing:

• USB or power connectors that fit only one way,
• Jigs or fixtures that prevent incorrect part orientation.

   Why it matters:
Error elimination leads to zero-defect processes, reducing or eliminating reliance on inspection and rework. It also tends to be a lot easier and faster, since we aren’t reliant upon an operator to notice that something is awry.

Level 2: Error Detection at the fact (aka Cannot Process)

At this level, systems are put in place to immediately detect errors before they result in defects. While not as powerful as prevention, detection still enables quick response and correction.

   Examples of Level 2 mistake-proofing:

• Alarms or indicator lights when errors occur,
• Digital counters that alert when steps are skipped or incomplete.

   Why it matters:
Immediate detection helps contain quality issues early and reduces the risk of passing defects downstream or, worse yet, customers.

Level 1: Error Detection after the fact (aka Cannot Pass)

This level is reactive—it doesn’t prevent or detect the error at the source but attempts to limit its impact by not progressing to the next operation, or to the customer. It’s considered the weakest form of mistake-proofing in Lean.

   Examples of Level 1 mistake-proofing:

• Final inspections before shipping (either to end Customer or the next process),
• Operator training to reduce human error.

   Why it matters:
Mitigation helps catch lingering issues, but it’s time-consuming and costly. The goal in Lean is to gradually           move away from this level toward prevention and detection.

Wicked Good Mistake Proofing in New England

Now, far be it from us to not put a distinctly New England spin on this concept, so let’s talk about Storrowing!

If you’re unfamiliar with the term, allow us to explain. “Getting Storrowed”, or “Storrowing”, refers to our beloved Storrow Drive in Boston, which has gained infamy over the years due to their low-clearance underpasses. Officially opened in 1951, for a good long while everything was fine. However, as Boston grew, traffic became (and still is!) a problem (when you see me next, ask me about the time I drove the wrong way down a one way street in the Fincancial District at lunchtime on a Thursday). As such, not only did the population grow as more and more people moved to the city, but so did the vehicles that people use to support that migration – specifically, delivery and moving vans. Which brings us to the issue at hand…

As the above story from CBS News Boston reports, “Storrowing” is a big problem, since modern-sized trucks simply do not fit under the overpass. In fact, some clearances are only 10 feet, which is less than some parking garages I’ve been in!

In an effort to prevent this from happening, the city put up signs at the entrances to Storrow Drive (and a few others, like Soldiers Field Rd. and Memorial Ave., (aka “the River Roads”)) to indicate that there were low clearance bridges up ahead. When that didn’t work, they put up more signs. Then they added flashing lights. Most recently, as this story from NBC10 Boston shows, new signs are being added at the same height as the overpasses in an effort to “alert” drivers that, if they hit that sign, that they’ll hit the overpass (which would be equivalent Level 2 Mistake Proofing). Having been piloted a few years ago at some entrances, Massachusetts Dept. of Conservation and Recreation (MassDCR) engineers report that the signs are an effective method to prevent overpass strikes, and hence why more are being added.

Source: Google street view

Here’s the problem: While they do convey important information, these signs don’t actually prevent trucks from hitting an overpass. They’ll certainly alert the drivers to that danger, but we’re also assuming that they notice hitting the sign. What if the radio is loud? What if they think that “thump” they just heard was a moving box falling over in the back? Lots can still go wrong…

Look at the picture above. Let’s assume that a driver does recognize that they hit the sign, and therefore can’t fit under the overpass up ahead. What are they suppose to do? There’s no “off ramp” that they can take. If they hit the sign, then stop, they’ll still snarl traffic with no way to fix it. Maybe they can back up and go into that parking lot, but then what? I once drove my extended cab pickup in Boston’s North End, and that thing barely made it around corners, let alone an entire moving truck!

I don’t know if you’ve ever driven in Boston, but it’s pretty nerve-racking. The roads make no sense. Where Manhatten, NYC was designed and thought out as a giant grid, Boston just sort of “is”. A “straight only” lane will turn into a “right turn only lane” within a matter of feet. Roads seldom meet at 90 degree angles. There’s always construction, and our fellow drivers are famously short on patience. Mix all of those in with the fact that someone who is likely new to the city (often a college kid or their parent) is driving a vehicle they aren’t familiar with on roads they don’t know, and it’s very easy to see how this can happen. They are just following their GPS, which doesn’t know what you’re driving, so it’s not sophisticated enough to route trucks a certain way and cars another. So – what to do?

I’ve long been an advocate for Level 3 mistake proofing – increase the clearance, and simply make this whole probelm go away. Modern underpasses have anywhere from 14′ to 16′ clearance for this very reason. Yes, it’ll cost a lot of money to alter the roadways, but it’s the only way to ensure that this problem goes away for good. Anything short of that, and we’re simply fooling ourselves. Let’s see what happens this September when people are moving in. I’m willing to bet we’ll see some Storrowing happen!