Correcting Human Errors the Japanese Way with Poka-Yoke

How a Simple Japanese Aphorism Helps Counteract Murphy’s Law and Contributes to Defensive Design

Among popular business axioms, perhaps none is more universally known than Murphy’s Law. Often quoted as “anything that can go wrong will go wrong,” the adage is used to describe the inevitability of error, particularly human error, in foiling the best-laid plans. However, simply giving in to Murphy’s Law without a fight guarantees failure. Thus, businesses have for decades sought to decrease the frequency of Murphy’s Law through complex training and management strategies. However, one of the most successful counteractions for Murphy’s Law comes in the form of an unassuming Japanese saying – one so unknown, in fact, that few outside of Japan have ever even heard of it. However, this humble idea is one of the core concepts of the Toyota Production System, which ranks among the most successful and emulated quality management systems (QMS) in the world. What, then, is the secret of this Japanese phrase… and how can companies apply poka-yoke (“fool-proofing”) to their own quality management systems?

Table of Contents

1. Getting Smarter: An Explanation of Poka-Yoke

2. How to Use Poka-Yoke; or, How Not to be a Fool

3. Preventable Versus Unpreventable Incompetence

4. Reducing Fallibility in Your Company

5. Additional References and Resources

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Getting Smarter: An Explanation of Poka-Yoke

First off: what does poka-yoke mean? Perhaps unsurprisingly, this playful expression originates from a game: the Japanese chess-like game called shogi. In shogi, players move tiles across a board while trying to capture the opponent’s tiles; as in chess, strategy and thinking multiple steps ahead are required for success. The term poka-yoke relates to these strategic thoughts. Specifically, it describes a move so bone headedly stupid that it would singlehandedly cause the player to lose the game. Therefore, although lighthearted, the sentiment of poka-yoke is the desire to avoid terminal errors caused both by the absence of thinking and foolish thoughts.

As applied to business philosophy, poka-yoke overlaps with the more well-known ideas of ‘idiot-proofing’ or defensive design. However, while these corresponding ideas primarily relate to how a product will be created and then used, poka-yoke also relates to idiocy prevention in production management and quality control. It also relates to the different concepts of inevitable human errors as contrasted to manufacturing defects. Poka-yoke doesn’t attempt to correct all human errors; that would be, as they say, a fool’s errand. Rather, it seeks to correct thinking processes to draw attention to common errors, and thus remove defects. It is, in effect, the creation of a check process that prevents a person from proceeding with a foolish thought (or a lack of thinking altogether) by gently reminding them of what they need to do to achieve a correct result. The concept is also known in industrial contexts as forcing function or behavior-shaping constraint – in other words, a behavioral and mental safeguard.

Poka-yoke might not be a familiar term to most English speakers, but examples of this concept are all around us. For instance, think about how you start your car every day. On most modern cars, you don’t just turn a key or push a button; you have to do something else to make it work. What is that something else? Pushing down on the brake pedal! This is a forcing function implemented by smart and safety-minded auto designers – because you don’t want a car to start rolling the second you turn it on. Thinking about the concept using this example, you can quickly realize just how widespread poka-yoke implementation is.

Given this example, perhaps it should come as no surprise that the poka-yoke idea was first proposed by Shigeo Shingo – an external consultant who worked with auto manufacturer Toyota to document the Toyota Production System. Shingo, who we highlighted in our earlier article about the 5S quality management philosophy, examined quality management from the perspective of modifying and adapting human behavior. In the case of poka-yoke, he particularly envisioned applications where manufacturing processes could be improved by ensuring a worker could not complete their task without first checking that something had been done correctly. In his influential book A Study of the Toyota Production System From an Industrial Engineering Viewpoint (1981), Shingo explained that the poka-yoke principle could be applied in a mass-production system through the use of forcing functions that required enhanced self-inspection by workers. For example, Shingo noted that, during the assembly process of a switch, workers often forgot to install the springs that needed to be included in the switch in order for it to work. To provide a forcing function that would remind workers and avoid defects, he designed a placeholder box that workers would put the springs into. He then reconfigured the assembly process itself so that the workers, rather than just putting the springs into the switch as a single action, would first put the springs into the placeholder, and then perform a second action to move them from the placeholder into the switch assembly. When workers did this, they would inevitably notice if any springs were still left in the placeholder, thus helping them recognize when mistakes had happened. This simple, self-directed inspection and correction was the basis for Shingo’s poka-yoke principle. In later years, he helped Toyota design much more complex means for reducing errors, including by automating the process. In another example, Shingo identified that workers sometimes incorrectly formed sheet metal parts by loading metal panels intended for the left side of a car part into the press intended to make the right-side panels. This was an easy mistake to make, because the left and right-side sheet metal parts were identical to each other, except for the position of a small hole in the metal (on the left sheets, the hole was to the left upper corner; on the right sheets, it was on the right upper corner). To address this, he redesigned the part press to have a notched switch that would automatically push through the hole on the right-side part; if the correct right-side metal panels were run through the machine, the switch would notch into the hole and all would proceed, but if a left-side panel were accidentally loaded, the switch would be pressed, halting the machine before the panel was bent. This clever design not only prevented mistakes, but also helped workers identify what to look for when loading the panels in the first place.

Shingo identified two strategies for implementing forcing functions: a control type function, which would prevent the manufacturing or usage process from continuing until corrected; and a warning type function, which would alert the user but not halt the process. The control type is generally the stronger and more useful function since it does not permit the mistake or dangerous activity to persist, but in certain circumstances, a warning type function (such as a flashing light or alarm) may be suitable. Shingo generally believed that mistakes which are easily correctable without danger or significant cost are suited to warning type functions, whereas all others benefit from a control type function, even if the resultant stoppage causes delays for workers or users. This is because the delay is likely to ultimately be less of a detriment to efficiency and expenditure than the possibility that uncorrected errors persist. Within the control type function, Shingo further broke down the specific applications into three possible methods: a contact method where the product is placed into contact with the manufacturing device and this contact measures whether a defect exists or a mistake has been made; a fixed value method where the proper manufacturing step or use of the product is dictated by a certain number of required movements; and a motion-step method where the movements or steps of manufacturing or use must be exactly followed. In Shingo’s applications of poka-yoke, control type functions generally used just one of these methods, but combining multiple methods is also possible.

With all these ideas condensed into his poka-yoke principle, Shingo worked with Toyota to remake its auto assembly lines. Their success was so substantial that Shingo would later state that poka-yoke could lead to “zero defects” for anticipatable errors. The principle would later go on to be implemented into Lean and Six Sigma production methodologies, and under various other names and interpretations, come to be included in quality management systems. However, relatively few people know the exact origins of poka-yoke, or just how comprehensive the principle’s application can be when extended outward through an organization, or even a society.

The next step in achieving poka-yoke’s benefits is considering how to implement it – both intellectually, and practically. Because the more you think about inadvertent absentminded thinking and foolish thoughts, the more you realize that these human failings (or human foolings, as the case may be) are just as pervasive in the world around us. Thus begins the ultimate challenge: to anticipate just how many failings might exist, to evaluate which ones are correctable via conscious action, and to accept the innumerable other failings that we won’t be able to correct, but that we can hopefully reduce to lesser relevance through our other cumulative efforts. Shingo’s broader philosophy, which was shared by others in Toyota, emphasized an empathetic and understanding approach to humanity within the context of efficiency and business. Therefore, he sought to show with his principle that since anyone can make a mistake, the best way of thinking about and dealing with mistakes is to not focus on individual blame or responsibility, but rather to gently correct human nature. In his aforementioned book, Shingo wrote:

“It is important to realize that there are two types of forgetfulness. Since we are not infallible, everyone forgets or overlooks things now and then. This is the first type of forgetting. The second is ‘forgetting what we might forget’ – when we forget to make sure that we have not overlooked something. To guard against this, we make checklists. Poka-yoke methods incorporate the function of a checklist into an operation, so we don’t ‘forget what we forgot.’”

How to Use Poka-Yoke; or, How Not to be a Fool

As the above section indicates, poka-yoke is not about preventing all failings. Instead, it’s about preventing a certain type of failure that is especially common, and which is especially difficult to otherwise prevent. This is the failure of unconscious foolishness, also known as absentmindedness. It has nothing to do with mental acuity or inherent intelligence, but it does have a lot to do with repetitive actions and processes, including other quality management processes, that can inadvertently lull a mind into committing unconscious errors.

Let’s return to the example of the car that won’t start without pushing down on the brake pedal. Think about the thousands of times you’ve probably started a car. How many of those times was your mind on something else? Something that had nothing to do with starting the car? Maybe you were thinking about what you needed to accomplish while being in the car. Maybe you were rushing. Maybe driving is so familiar that you simply drifted off into unrelated thoughts, since starting the car requires minimal attention and thinking. In any case, imagine that you slipped away from thinking about starting the car, even just once out of those thousands of times. What would it have taken for the car to have suddenly lurched forward and hit something? That’s why the forcing function with the brake pedal is there. It’s not because if you don’t push the brake, you’re a fool; it’s because anyone can be a fool, in the right (or wrong) set of circumstances.

Now, let’s think about how this same behavior of unconscious thought can impact quality management in a business. Let’s say your company has a system that consists of many repeated quality checks and actions. Under normal ideal circumstances, it would work great. But what about normal real circumstances? What about when a quality manager has zoned out? On a tired day, your eyes may glaze over, you may read and re-read the same sets of notes and okay something that is completely wrong. Or, what about a worker in a manufacturing operation? It doesn’t take much to imagine how easy it would be for them to be so comfortable around their machinery that they forget to take all the safety precautions. It doesn’t take many of these incidents to turn into a large and dangerous problem.

All of these reasons are why poka-yoke is useful: it requires planners to anticipate what might inadvertently happen. Just from that first step alone, your organization has already taken a step away from foolishness. This is because anticipating requires conscious and considerate thoughts, and those are never foolish. Next, designing and implementing a strategy involves taking deliberate actions, and once these occur, they may spur considerations for additional inadvertent foolishness that hasn’t been identified yet. Finally, by educating workers and managers about the steps implemented, increased awareness and vigilance against foolishness can be cultivated in the workforce. It’s important for employees to understand that these safeguards aren’t a judgement against them, but rather an acknowledgement that we are all fallible and can use a helpful reminder when we are imperfect. Indeed, worker representatives now often require poka-yoke safeguards as part of employee protections and rights.

The best way to use the poka-yoke principle is to treat it as a comprehensive thinking and communicative exercise. Sit down with the managers, the workers, and the executives, and consider everything that could go wrong in ordinary practices. This isn’t about anticipating extraordinary scenarios, but identifying the failures of the mundane. Being open and non-judgmental when considering these unthinking mistakes is crucial, while hubris is the direct enabler of foolish failures. It’s best to assume that, as Murphy’s Law postulates, any foolish error that can happen will inevitably happen, with each and every one of us invariably playing the fool.

Preventable Versus Unpreventable Incompetence

This raises perhaps the most important question in considering the scope and aims of poka-yoke implementation: what is conceivable and preventable failure, and what is not? As defined above, conceivable failure is a mistake that can (and likely will) happen to anyone under a given set of circumstances (such as absentmindedness or fatigue). This is preventable incompetence. Unpreventable incompetence, on the other hand, is foolishness so deep that no amount of preparedness can remedy it. Contrary to what you might think, unpreventable incompetence is not deep and irreversible stupidity in individuals. Rather, it is institutional stupidity that is innate to humanity itself as a group, and is enabled by the universal laws of chaos. In other words, foolishness that is so foolish, and so universal, that everyone regards it as normal, is beyond the scope of any quality management strategy.

Unpreventable incompetence is such a vast problem as to go unrecognized and unacknowledged in most quality management systems. By its nature, quality management sets as its goal the total prevention of errors, and measures success by how closely it can adhere to that goal. Unpreventable incompetence is essentially the force that prevents total error prevention from being possible. The reason for this is that unpreventable incompetence is formed via the accumulated actions and reactions of humans to all non-anticipatable scenarios which can occur, through chance or the results of institutional assumptions, and is therefore simply too large of a problem for the mind to assess through the narrow filter of personal failure and foolishness. But it is for this very reason that unpreventable incompetence should be a vital component of quality management considerations – after all, in battle, it is important to know who your enemy is. And it's also critical to understand that, as unpreventable as unpreventable incompetence may be, its effects are often triggered by preventable incompetence, creating cascading foolishness which can often be far worse.

Utilizing another example to understand the differences between preventable versus unpreventable incompetence, we can see how the two are also interrelated, and how taking steps to mitigate preventable incompetence can reduce the likelihood that unpreventable incompetence will occur. In the original Jurassic Park (1993), the scenario which endangers the park and its human guests stems from a vast multitude of both preventable and unpreventable incompetence incidents. Let’s divide them into categories for easy consideration:

Preventable:

• Poor vetting of employees / hiring practices

• Designing security and computer systems that lack adequate backups

• Building in a storm-prone area

• Reviving dangerous dinosaur species

• Failing to anticipate selfish human behaviors

Unpreventable:

• The desire to play God

• Using science that is not understood

• Inability to respond adequately to chaos

Now, of course the question is: if the preventable foolishness of Jurassic Park’s ownership had been prevented, would the unpreventable incidents have also happened? Perhaps the unpreventable incompetence would have eventually doomed the park, but within the context of what happens in the story, smarter choices regarding the preventable incompetence areas would have protected the park’s management and saved the lives of all the guests on the island. But instead of taking the poka-yoke principle to heart, which could have prevented disaster even if applied to just one of the preventable incompetence areas, cascading foolishness instead occurred, ultimately untapping the results of the unpreventable incompetence areas as well.

As a concept, cascading foolishness is essentially predicated on the idea that single isolated failures stir the pot of unpredictability and induce further bad decisions, or at the very least, expose problems that might otherwise be avoidable. In motorsports, this idea is epitomized by a saying: “cautions breed cautions.” In other words, once an accident has happened on track, it’s more likely that other accidents will also happen, in part because the drivers are now on edge, nervous, and prone to poor decision-making. Rather than an accident highlighting where things could go wrong and thereby warning people against mistakes (the so-called ‘cautionary tale’ idea), cascading foolishness suggests that once something starts to go wrong, it breaks the dam holding back all the other idiocy. Thus, the poka-yoke principle stresses preclusion, coupled with awareness reminders, as the tool to both warn and prevent.

Returning to the idea of preventable versus unpreventable, some companies may question whether accidents (either within the company or within the consumer groups using the finished products) can be prevented when chaos and unpredictability make a majority of usage scenarios difficult to anticipate. This is of course a question for risk analysis and overlaps with liability concerns, as well as quality management. But the clear dividing line from a poka-yoke perspective is whether the mistake can be defined as simple foolishness or oversight. If the error is something that a worker or consumer could conceivably do within the context of normal usage, or if it is something so innate that the risk is inherent whenever usage occurs (such as the necessity of using the brake when starting a car), then it qualifies as poka-yoke. If the mistake requires the significant intervention of non-user forces, such as the forces of chance and chaos, then it is not preventable via poka-yoke implementation, and should instead be mitigated by other strategies. Ultimately, poka-yoke works best when applied to known knowns, and can also be used in some cases for known unknowns; it is not suitable when applied to unknown unknowns.

Reducing Fallibility in Your Company

This brings us to our conclusion, and the most important part of this article: how can poka-yoke directly be used to reduce fallibility? The best approach is a holistic one: invite all members of the organization to self-analyze and use observational techniques to identify where problems are likely to occur in normal operations. Next, ask everyone to be honest and self-critical when identifying how they are likely to make mistakes. Some of the discoveries at this stage will indicate behavioral changes that can be implemented, such as ensuring adequate rest and limiting working hours. Others will be environmental, such as creating a distraction-free work environment. The most obvious poka-yoke implementations, however, will be in the creation of physical forcing functions. For example, in a piece of manufacturing equipment that has an exposed mechanism which could injure a worker, a forcing function would be an activation switch that requires the worker to use both hands simultaneously to activate the machine, thereby preventing the worker from inadvertently placing their hand in the mechanism. Once these behavioral, environmental, and physical implementations have become apparent, poka-yoke can be taken a step further by encouraging the organization to consider non-obvious errors that might occur. This is the step between remedying known knowns and considering known unknowns. This is also the point at which quality management moves beyond the work done directly within the organization and extends to the products which the organization provides. For example, designers, workers, and managers alike should anticipate errors that customers may make while using the products, and implement forcing function mitigations directly within the product designs. This can be achieved not only via design work and analysis of anticipated product usage, but by communicating with and including the feedback of workers involved in the manufacturing process, since ideally, they will have already been involved in practicing poka-yoke themselves.

In its broadest application, poka-yoke should include everyone connected to the product and process, including customers, since this vast accumulation of considered potential error occurrences will anticipate the broadest possible range of likely errors. Via this, and hypothetically extended to a vast scale, it may in fact be possible for even unpreventable errors to be identified and corrected, since the accumulated ‘group of fools’ would be so enormous that even these incalculably complex group mistakes could at last be seen, and corrected. But short of this utopian dream which borders on wishing for the perfectibility of humanity, poka-yoke should extend as broadly as is practical within a given context, since the wider its scope, the more effective it will be in inducing self-identification and awareness of errors. Whether that means extending within a given workspace, an entire organization, or a customer base depends on the complexity and goals of the implementing organization.

As we look to the future of poka-yoke, quality management and efficiency experts have postulated that digitally-enhanced poka-yoke methods may become standard in the coming years. For example, workers on an assembly line may be able to utilize augmented reality glasses to see poka-yoke checklist overlays while they work, or benefit from workplace projections or light-based instructions that shine onto or from their workspace. Improvements in AI might also help identify and mitigate errors. An example of this might be using cameras to capture assembly line data, which is then interpreted by an AI that provides feedback to workers. As emerging technologies are implemented into both manufacturing and usage, poka-yoke’s versatility will enable it to be utilized in a variety of new ways, and will open the door for clever quality managers to further improve results and reduce errors.

The author Douglas Adams, renowned for his Hitchhiker’s Guide to the Galaxy series, famously opined that “a common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools.” His humorous summation of Murphy’s Law may seem to undermine the viability of poka-yoke’s goals, but in fact, the sentiment establishes exactly why poka-yoke is valuable. The point of including poka-yoke within a quality management system is not to make something completely foolproof, for indeed, the complete fools who can undermine it are all humans collectively, and nothing short of a collective application of poka-yoke could undo that. Rather, the point is not to design a system which requires or expects perfection, but to instead recognize the fool within. When we all acknowledge our own foolishness, and forgive ourselves for being human, we can begin more adequately to work toward correction… and maybe someday, achieving that utopia.

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Authors

Yelena Rymbayeva is the Chief Marketing Officer of QMS2GO. A veteran marketing professional with experience in software, product development, and entertainment-focused startups, she has written extensively on business organizational best practices, efficiency strategies, and quality management system implementation, with an emphasis on small/mid-sized manufacturers and technology development companies.

Nicholas R. Zabaly is the Editor-in-Chief of QMS2GO’s research and knowledgebase operations. An experienced researcher and technical writer, he has worked closely with the company since its foundation and serves as its lead article writer. Specific to the content of this article, he has experience working with Japanese companies and has knowledge of the Japanese language.

Additional References and Resources

[1] American Society for Quality (ASQ) – “What is Poka-Yoke (Mistake Proofing)?” – https://asq.org/quality-resources/mistake-proofing

[2] The Quality Portal – “Poka-Yoke or Mistake Proofing: Overview” – https://thequalityportal.com/pokayoke.htm

[3] Productivity Press – “A Study of the Toyota Production System From an Industrial Engineering Viewpoint – Revised Edition” (Shigeo Shingo, 1981; revised 1989) – https://books.google.com/books?id=RKWU7WElJ7oC&printsec=frontcover&source=gbs_ge_summary_r&cad=0#v=onepage&q&f=false

[4] Harvard Business Review – “Poka-Yoke is Not a Joke” (2010) –  https://hbr.org/2010/02/my-favorite-anecdote-about-des

[5] Industry Week – “Intelligent Poka-Yoke: When Lean Manufacturing Meets Deep Learning” (2018) – https://www.industryweek.com/technology-and-iiot/article/22025815/intelligent-poka-yoke-when-lean-manufacturing-meets-deep-learning