Poka yoke is Japanese for mistake proofing. It is a way to reduce errors and mistakes by designing the product so that it cannot be assembled or used incorrectly. I find this extremely useful, not only in manufacturing but also in many instances in daily life where poka yoke prevents me from making mistakes. In this post, I would like to talk about poka yoke, and the two fundamentally different directions poka yoke can take.
Introduction
Poka yoke (ポカヨケ) started out as baka yoke (馬鹿ヨケ, or idiot proofing). However, people soon complained that management considered them idiots, and hence needed idiot-proofing. The term was then changed to the more politically correct mistake proofing. It is loosely related but different from Jidoka, albeit they are sometimes confused since both improve quality. Jidoka is the idea of stopping a process automatically if there are irregularities, and hence to detect errors right when they happen. Poka yoke is to prevent these errors from happening in the first place. Clearly, preventing an error is better than detecting it right away, which is still better than detecting it much later, which is still better than the customer detecting it for you.
Examples
There are plenty of examples in everyday life where poka yoke is used to prevent mistakes. In manufacturing, it is mostly to prevent incorrect assembly or, generally, manufacturing, but in daily life it is more to prevent incorrect usage. You are probably familiar with a SIM card for your mobile phone. It is a rectangular card with some electric contacts. To make sure you do not insert the card incorrectly, one corner of the card is missing. The matching slot in your mobile phone has one corner filled out. Hence, instead of four possible ways how you could add the card, it can be added only one way.
Another example is the USB-A connector. It will fit in the corresponding USB-A port only one way, and it is not possible to plug it in incorrectly with normal use.
Yet another example at least some of you are familiar with is the connector of a desktop computer power supply unit. This is actually a manufacturing example, since you probably see them only when you assemble a computer. There are typically numerous different connectors having between four pins and twenty or twenty-four pins. Due to their different sizes, it is usually not possible to connect the wrong connectors.
Yet another example is the ATM machine in most countries. To get money, you insert your bank card, enter your pin, select the amount you want (or can afford), and then THE ATM FORCES YOU TO TAKE THE CARD before it gives you money. This way it is much less likely that you forget your card in the ATM. Few people will forget the money…
Overall, there are many different examples of poka yoke in daily life, and even more on the different shop floors. They make it impossible (or at least difficult) to do a wrong or flawed action. The way these work can be grouped into two groups:
Permit-Only Correct Action
One way is to permit only the correct actions. Above are examples for this. It is simply not possible to use these devices incorrectly.
However, there is a second way to do poka yoke:
Tolerance for Variety of Actions
Rather than limiting the user to correct actions, the device may allow a multitude of different actions without defects. Take, for example, the SIM card from above. It cannot be added incorrectly, hence the contacts always make contact. You actually own a few other cards that also include contacts and a chip: your bank cards and credit cards. Some ATMs force you to put the card in the correct way and refuse to accept the card if it is incorrectly. This would be Jidoka, where the ATM detects an incorrectly entered card. However, it should be technically not a problem to design an ATM that accepts the card in multiple orientations. No matter which way you enter the card, the ATM can read it.
Similarly with the USB-A connector above. I often need multiple tries (and, embarrassingly often, more than two) to enter the USB-A connector into the port. This is much easier with the newer USB-C connector, which can be entered both ways and it will work. Accepting the connector either way does not force the user to conform to the device. You have even more options with the headphone jack, which has radial symmetry and will fit in any rotation.
If you are a bit older, you may also remember the multitude of different adapters for charging your electronic devices. You had a whole drawer full of different charging cables, most of which you forgot what device they were for. When looking for a cable, you also needed to figure out which one it was. And good luck borrowing a charger from a friend or colleague, it probably did not match yours.
Admittedly, the design (usually) kept you from connecting the wrong charger. But it was still a major pain to find the right one. Thanks to the European Union, the USB-C connector is becoming the standard for all devices, and you can finally throw out your drawer of cable junk.
The ATM forces you to take the card first, but with a modern wireless payment system using your mobile phone or a touch card, you will never put the card or phone in the machine anyway, and don’t need an additional poka yoke so you don’t forget your card.
Summary
Overall, having a system that accepts multiple different actions or uses the same part for different products (like the USB-C charger) makes it a lot easier and less wasteful for the user or operator. If you can do your poka yoke not by forcing the user to conform to a certain action but instead accept a multitude of different possible actions, it will be much easier for the user.
On the other hand, depending on your system, it may be more expensive, and may not worth it. For example, the SIM card could be added four different ways if there was no cut corner. You could add connectors in a mobile phone for all four different positions, and the user would never have to worry about positioning the card correctly. Unfortunately, additional connectors cost money, add weight, and add another potential defect. When did you last change your SIM card? Overall, the effort may not be worth it. But in other examples it may be, like with the many old phone chargers replaced by USB-C (and thank you again to the EU for doing this!). Now, go out, make your system tolerant to multiple actions, or—if it is not possible—enforce only the correct action, and organize your industry!
Understanding the basic principles for achieving waste free fleible-flow & zero defects is the foundation of lean operations. I had an opportunity in 1989 to discuss Poka-Yoke with Sensei Shingo himself. Below are extracts from my notes on his comments, and my own experiences over the last 30 years. Initially he chided me for talking about it as an individual technique, & said it should be seen as the tool for implementing his system of ‘source inspection’ & guaranteeing zero defects. —
He explained that traditional ‘long cycle’ inspection systems wait until an error in action produces a defective item, the defective item is then found by inspecting the output. His concept of source inspection uses the ‘short cycle’ inspection system. In this system the action itself is checked 100% using mechanical means. If an error occurs, immediate action is taken to correct it before a defect is produced. With this methodology we can guarantee zero defects to the final customer.—
The basic system is simple;
The PY methods/devices should be designed to detect deviation from the standard actions & outputs required to satisfy the customer’s requirements.
This can be done in three ways; a) Physical contact. b) Fixed values. c) Motion steps.
In some cases at the original design stage the part can be made a PY device by ensuring it can only be assembled/used in the correct way.—
They should also check for deviation in the 3M’s of actions & items;
Missing. Action or item not there. Misplaced. Action or item there, but in wrong position.
Malformed. Action or item is there but wrong, size, shape, colour, temp etc.
When designing Poka-Yoke devices they must check for specific deviations in the 3M’s using; a, b & c. This can be done with a ‘what can be’ 3 M’s analysis.
The Poka-Yoke device should then;
1) Control the operation. Stop the process when an error or defect occurs.
2) Warn the operator. Signal to the operator that an error or defect has occurred.
They should be applied at the following check points;
1) The source action. (source check) This is the ideal as it gives zero defects.
2) Output of the action. (self check) . This is our second choice as the output will be defective if the PY device is activated, but it will not be passed to the internal customer.
3) Before the next process. (Successive check). At this stage the item will be defective if the PY device is activated, but it cannot go to the final customer.
With this system in place it is now possible to consistently achieve;
‘Zero Defects in our activities & production processes’. —
If applied to safety it is possible to achieve ‘Zero Accidents’. I do not understand why this methodology is not more widely used in this area. —
The most impressive example of Shingo’s system I have experienced was on an assembly line for inlet manifolds in Japan. We were allowed to work on the line & challenged to produce a defective assembly. It was impossible to produce one, & we had some very talented people trying. —
Once our front line people understand this system they become some of the best designers of Poka-Yoke devices. —
Poka-yoke should be seen as the device for implementing Shingo’s zero defects system.
The goal is to identify deviation from the desired conditions or actions in any situation.
A good example is the selector stick on an automatic car gearbox. If the stick is not in the park position the poke-yoke switch is not activated & the engine will not start. Zero defects in all situations. Shingo pointed out to me that this would be impossible to achieve with statistical techniques.
The idea seemed novel when introduced here 40-odd years ago – but only in the context of quality. Fail-safe mechanisms – obviously in the realm of safety – have been around for a long while.
Building on Sid’s comments, another aspect is that devices can operate in 3 ways:
– cannot accept defect
– cannot build defect
– cannot ship defect
(I know which I prefer!)
Hi Steve, good take on the different ways to avoid defects. I agree with your preference 🙂
Great read! It’s cool how Poka Yoke isn’t just a factory thing but also sneaks into everyday life to stop us from messing up. I never thought about it in terms of letting us do different things without goofing up versus just allowing the right moves. Makes you think twice about how stuff’s designed, huh? Thanks for sharing!
Most people who did their own car maintenance heard about relays. A relay is an electric switch that closes or opens a circuit using a solenoid. a typical 4 pin relay will have two main power pins and two signal pins. In the late 90s and early 00s, Japanese manufacturers keyed individual relays so that each one would only go into one slot in the fuse block even if the pin layout was otherwise identical.
Sounds great until you are working on older car and a relay with a matching key isn’t readily available. a lot of the times, it’s not possible to just swap a suspect relay with a different from from a less essential function for testing. Sometimes I joke with customers that their horn is actually a built in bench tester for fuses and relays. If you press on the center of your steering wheel and gear a loud noise, the relay is good and the fuse is good!
here’s one example of keyed Toyota/Lexus relays.
https://www.ebay.com/itm/355122125418?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=qQjV5OYPQ9W&sssrc=4429486&ssuid=hjGDds4bTsS&var=&widget_ver=artemis&media=COPY
notice how they have different colors and key notches.
GM, on the other hand, took a much better approach and made a lot of their 4pin relays work no matter what orientation it was inserted in.
ACDelco GM Original Equipment 13500114 Multi Purpose Relay https://www.amazon.com/dp/B08NMVWKCS?starsLeft=1&ref_=cm_sw_r_apan_dp_WSB62CS1JAP66C2JHXH8_2
if you look up the electrical diagram. main power pins are on opposite corners. same with the signal pins. if you were to crack one open, you’d see the solenoid plunger plus a contact on both sides and the main circuit will close regardless of signal polarity.
Hello Andrey, excellent example. Toyota relays can be put in only one way, GM in any way. In this case, I think I prefer GM.
In Vancouver, Canada, thé local transit authority had transit users suffer through several unfortunate years when their passes could be entered into the readers in one of four different orientations (rectangular cards that required a specific face up and the stripe on a specific side) but only one worked. It slowed the bus loading process by 15-30 seconds per passenger, on average, which became a significant factor in busses running late. This was only fixed when they replaced the entire system, changing the type of fare card and using proximity readers where the orientation didn’t matter. I gritted my teeth every time I boarded a bus.
hello Hugh, great example on how NOT to do it. Glad it is fixed now 🙂
Hi Christoph,
Your studies and blogs have many great observations and directions for all lean-enthusiastic engineers and leaders. Lean is now global word for Toyota manufacturing systems almost more than a quarter century but there are still a lot to learn from Japanese culture and Toyota practices literally.
Poka yoke is another one. I do prefer use Japanese words in business while I am applying becasue it is deeply linked with its motherland root somehow. Passion for journey in excelence is kind of a Toyota culture. Poka yoke is tool for reaching zero defect. But again like all other lean mechanism, it has to be worked in harmony with many other little lean tools/meachanism to be more effective.
Poka yoke itself needs these tools primarily. Such as; jikotei kanketsu, hatsumono-owarimono.
Simply saying, jikotei kanketsu is not to accept, not to produce, not to transfer the defects in one process which is fully supportive for poka yoke. It also configures where and how to install poka yoke in the process flow. Once you have jikotei kanketsu study for whole process flow thus you know where to structure defect-trap (poka yoke).
Second point is that hatsumono-owarimono which is shift-end or shift-beginning control points for product and process CTQ points. You should also check poka yoke applications are functional or not effectively, otherwise you will have leakage which ends up with high scrap or big complaints later on.
Poka yoke is a concept and mind-set for each shop-floor member to follow and monitor the process, not something that you install and forget.
Thank you so much for raising this crucial lean tool!