In this issue we will be discussing another well-known Japanese concept –
Poka-Yoke.
Haven't we often blamed ourselves for not averting a disaster? Often! Ironically, this strikes us only
after the disaster has played havoc. Why does this happen? Is it our inability to foresee mistakes and
misread disasters or is it just a case of take every day as it comes? A debatable question, be it the
Chernobyl disaster or the recent accidental explosion of the Columbia space shuttle.
Phenomenal failures are a result of system variability, faulty design and non-conformance, eventually
leading to disasters, involving huge financial and moral setbacks apart from manufacturing/equipment
down time.
How can we avoid these problems? The most effective way to avoid is to detect and eliminate the root
cause of the problem. Poka-Yoke offers an effective way of achieving this objective.
Happy Reading!!
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Poka Yoke: Techniques to Prevent Defects
Abdel Ahmed PhD
The European Centre for Total Quality Management
University of Bradford, School of Management
Emm Lane, BD9 4JL, UK
a.m.ahmed1@bradford.ac.uk
Poka Yoke is a quality management concept developed by a Matsushita manufacturing engineer
named Shigeo Shingo to prevent human errors from occurring in the production line. Poka yoke (pronounced "poh-kah yoh-kay") comes from two Japanese words - "yokeru" which means, "to avoid", and
"poka" which means "inadvertent errors." Thus, poka yoke more or less translates to "avoiding inadvertent errors".
Some people as "fool-proofing" sometimes refer to Poka yoke in English. However, this doesn't sound
politically correct if applied to employees, so the English equivalent used by Shingo was "error avoidance." Other variants like "mistake proofing" or "fail-safe operation" have likewise become popular.
The main objective of poke yoke is to achieve zero defects. In fact, it is just one of the many components of Shingo's Zero Quality Control (ZQC) system, the goal of which is to eliminate defective products.
Poka Yoke is more of a concept than a procedure. Thus, its implementation is governed by what people think they can do to prevent errors in their workplace, and not by a set of step-by-step instructions
on how they should do their job.
Poka yoke is implemented by using simple objects like fixtures, jigs, gadgets, warning devices, paper
systems, and the like to prevent people from committing mistakes, even if they try to! These objects,
known as poka yoke devices, are usually used to stop the machine and alert the operator if something
is about to go wrong.
Anybody can and should practice poka yoke in the workplace. Poke yoke does not entail any rocket
science; sometimes it just needs common sense and the appropriate poka yoke device. Poka yoke
devices should have the following characteristics:
1. useable by all workers;
2. simple to install;
3. does not require continuous attention from the operator (ideally, it should work even if the operator is not aware of it);
4. low-cost;
5. provides instantaneous feedback, prevention, or correction.
Of course, error proofing can be achieved by extensive automation and computerization. However, this
approach is expensive and complicated, and may not be practical for small operations. Besides, it
defeats the original purpose of poka yoke, which is to reduce defects from mistakes through the simplest and lowest-cost manner possible.
Poka yoke is at its best when it prevents mistakes, not when it merely catches them. Since human
errors usually stem from people who get distracted, tired, confused, or demotivated, a good poka yoke
solution is one that requires no attention from the operator. Such a poka yoke device will prevent the
occurrence of mistake even if the operator loses focus in what she is doing.
Examples of 'attention-free' Poke Yoke solutions:
1.
A jig that prevents a part from being misoriented during loading
2.
Non-symmetrical screw hole locations that would prevent a plate from being screwed down
incorrectly
3.
Electrical plugs that can only be inserted into the correct outlets
4.
Notches on boards that only allow correct insertion into edge connectors
5.
A flip-type cover over a button that will prevent the button from being accidentally pressed
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Three levels of Poka-Yoke:
1.
Elimination of spills, leaks, losses at the source or prevention of a mistake from being committed
2.
Detection of a loss or mistake as it occurs, allowing correction before it becomes a problem
3.
Detection of a loss or mistake after it has occurred, just in time before it blows up into a major
issue (least effective)
Although many techniques have been developed to prevent or control mistakes, most of these techniques are relatively ineffective. Effective mistake proofing cannot be developed without a sound understanding of the true characteristics of mistakes. A mistake occurs when a required action is not
performed or is performed incorrectly, a prohibited action is executed, or information essential for an
action is not available or is misinterpreted.
There are a number of characteristics associated with poka yoke techniques that makes it more effective than other techniques. This includes:
1.
Mistake proofing requires 100% inspection. It is impossible to detect and control rare random
events with sampling inspection. Since, 100% traditional inspection is too expensive and not
100% effective in detecting nonconforming product, mistake-proofing methods based on
poka-yoke are essential and the only practical solution.
2.
Mistake proofing must be inexpensive. Because mistakes are rare events and many different
types of mistakes must be controlled, companies cannot afford to spend large sums of money
on each mistake-proofing device.
3.
Many mistake-proofing devices are needed. Toyota has an average of 12 mistake-proofing
devices at each workstation.
4.
Outcome intervention is best. The best mistake proofing physically prevents errors or detects
when a mistake is about to occur or has occurred. Thus, these techniques intervene to block
undesired outcomes rather than controlling casual factors.
5.
Prevention is better than detection. Preventing mistakes is better than detecting mistakes,
which is better than detecting defects. If a mistake is not detected until a defect is generated,
rework will be required or the hardware must be scrapped. Thus, where possible, it is always
better to detect or control the mistake before a defect is generated. Similarly, there will be less
wasted effort if mistakes are prevented rather than detected.
6.
Control, Shutdown, or Warn. Because resources may be wasted if a process is shutdown, it is
better to control mistakes. If a process is shutdown, however, the problem must be addressed
to proceed. Hence, shutdown provides a more positive control of mistakes than warnings,
which can be ignored.
7.
The Most Important Quality Initiative. Only mistake-proofing effectively controls mistakes. Significant quality initiatives other than mistake proofing have marginal impact on the customer
perception of quality since mistakes are the dominant source of customer problems.
Poka-Yoke: A Misunderstood Concept
Poka-Yoke is a Japanese methodology for mistake proofing to avoid non-conformities from entering
into processes. It allows defect detection and elimination at the source. It can also be used as a continuous improvement tool.
Shigeo Shingo introduced the concept of Poka-Yoke in 1961, when he was an industrial engineer at
Toyota Motor Corporation. The initial term was baka-yoke, meaning 'fool-proofing'. In 1963, a worker at
Arakawa Body Company refused to use baka-yoke mechanisms in her work area, because of the
term's dishonourable and offensive connotation. Hence, the term was changed to Poka-Yoke, which
means 'mistake-proofing'.
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Poka-Yokes are mechanisms used to mistake-proof an entire process. Ideally, Poka-Yokes ensure
that proper conditions exist before actually executing a process step. Thus preventing occurrence of
defects in the first place. Where this is not possible, Poka-Yokes perform a detective function, eliminating defects in the process as early as possible.
Many consider Poka-Yokes as limit switches, optical inspection systems, guide pins, or automatic
shutoffs that the engineering department should implement. This is a very narrow view of Poka-Yoke.
These mechanisms can be electrical, mechanical, procedural, visual, human, or any other form that
prevents incorrect execution of a process step. Poka-Yokes can also be implemented in areas other
than production such as sales, order entry, purchasing, or product development where the cost of
mistakes can be as much as on the shop floor. The reality is that defect prevention, or defect detection
and removal has widespread applications in most organisations.
The Centre for Excellence in Operations (CEO) has developed a Poka-Yoke framework to help its
clients understand, the various classifications and applications of mistake-proofing mechanisms. Given
below is a brief overview of the framework:
Prevention-Based Poka-Yokes:
Prevention-based mechanisms sense an abnormality that is about to happen, and then signal the
occurrence or halt processing, depending on the severity, frequency or downstream consequences.
There are two approaches for prevention-based Poka-Yokes:

Control Method: This method senses a problem and stops a line or process, so that corrective action can take place immediately. Thereby it avoids serial defect generation. An example
of this is an assembly operation wherein, if one of the components is found to be missing before the actual assembly step takes place, the process shuts down automatically. Another example is an incomplete sales order, which cannot be released for production until a true manufacturability configuration is defined.

Warning Method: This method signals the occurrence of a deviation or trend of deviations
through an escalating series of buzzers, lights or other warning devices. However, unlike the
control method, the warning method does not shut down the process on every occurrence. It
is used when a bandwidth of acceptance exists, for a process. An example could be pressurising a vessel or a filling operation, in which the results need not be exactly the same. Although the process continues to run, the Poka-Yoke signals the operator to remove a defect
from the line, or make necessary adjustments to keep the process under control.
Detection-Based Poka-Yokes:
In many situations, it is not possible or economically feasible to prevent defects. This is particularly so
where the capital cost of the Poka-Yoke mechanism, significantly exceeds the cost of prevention. In
these situations, defects are detected early in the process, preventing them from flowing to downstream processes and multiplying the cost of non-conformance. The three categories of detectionbased Poka-Yokes are as follows:

Contact Method: This method detects any deviation in shape, dimensional characteristics or
other specific defects, through mechanisms that are kept in direct contact with the part. A
subset of this category is the non-contact method, which performs the same function through
devices such as photoelectric cells. An example of this might include a chute that detects and
removes upside-down or reversed parts, or an in-line gauge that removes dimensional defects
and reroutes them to a defect lockbox.

Fixed Value Method: This method is used in operations, where a set of steps is sequentially
performed. The fixed value method

employs automatic counters or optical devices and controls the number of moves, rate and
length of movement besides other critical operating parameters. In this case, mechanisms are
usually built into progressive stamping, welding, Systems Manufacturing Technology (SMT),
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and automatic insertion equipment. Sometimes this is referred to as odd part out method.
Fixed value also includes critical condition detection (pressure, temperature, current, etc.)
through electronic monitoring devices.

Motion Step Method: This method ensures that a process or operator does not mistakenly
perform a step that is not part of the normal process. An example of this is colour coding of
electronic components on drawings and totes to prevent using mixed or incorrect parts. Another example is a visual to assist customer service representatives, in providing the right literature sets for various products.
The best Poka-Yoke in the world is a robust design. Many of the needs for Poka-Yokes are attributable
to poor designs and/or unrepeatable processes. The second-best Poka-Yoke in the world is education
and awareness. The automotive industry is a leader in this area, with the use of its Advanced Product
Quality Planning (APQP) guidelines and supplier development programmes. Companies such as
Motorola, Allied Signal and General Electric are also ahead in this area, because they invested in their
Six Sigma Black Belt programmes, and have taken them upstream into the new product development
process.
Building a Strong Foundation
The Poka-Yoke philosophy requires a strong foundation in TQM (Total Quality Management). First,
organisations must learn to be customer focused. As Tom Peters once said, "The customer comes
first, second, third, fourth, fifth, and there ain't no sixth! "
Second, organisations must promote quality ownership at the source, and they need to ensure proper
investment in their people, which enables them to be truly empowered. Third, a clear distinction has to
be made between good versus bad quality.
Fourth, organisations must embrace the PSP philosophy: Pre-, Self-, and Post-Inspection at the
source. Last, Poka-Yokes require real-time feedback and corrective action. These are the building
blocks of an effective Poka-Yoke effort.
Mistakes happen in organisations for many reasons, but almost all of them can be prevented, if people
make the effort to identify when problems happen, define root causes, and then take the appropriate
corrective measures. The objective is to prevent, or at least, detect and weed out defects, as early as
possible in the process. The use of simple Poka-Yoke mechanisms and other safeguards can help
prevent mistakes from leading to catastrophic events.
Case Study
A Poka-Yoke Construction
Often projects are delayed due to non-conformance and defects. These in turn stem from process
variability. However, few industries can match the variability in construction industries.
For instance, a survey conducted in Norway in 1998 revealed that non conformity, defects, mistakes,
wastage and alterations account for 10% of the total project costs, not to mention the time wasted
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during rework and re-construction. The need to ensure a mistake free/fool proof project has never
been so forthright. So has been the need to incorporate best practices like Poka-Yoke.
Most companies install Poka-Yoke devices to detect the root cause of failure. Such devices allow suspension of the process where a defect is detected. Consequently, necessary corrective measures can
be taken. Nevertheless, just installing Poka-Yoke devices cannot prevent defects/non-conformities. It
is only thorough dedication, constant appraisal and learning that an organisation can improve the responsiveness of the Poka-Yoke devices.
The application
In order to devise a streamlined approach to mistake proofing in construction, researchers carried out
six case studies at construction sites across Brazil and England. They used devices like video recorders, cameras, work flow charts and approaches such as performance monitoring and interviews to
determine the effects of variability at construction sites.
As the project progressed, each stage was compared with its ideal and desired results. This approach
was based on the thought that - "If your ideal and practical progress coincide then the process is flawless/ideal". To ensure that the goal of process improvement was not lost, the ideal result was set so it
could be implemented practically.
Excerpts from the case
The results were strikingly similar to what researchers had expected. Massive work in progress was
observed at all construction sites. Furthermore, improper planning and inefficient utilisation of the labour force characterised the work. Workplace sampling indicated dissimilar working trends. This was
due to lack of a standardised approach. Photographs taken at the construction site supported these
facts.
The plus points
As a validation to the researchers assumptions, Poka-Yoke devices wherever used improved the efficiency of the project and reduced non-conformity, which was a primary concern at few sites.
One such Poka-Yoke device prevented the load elevator from moving until the loading doors were
closed. It was often observed that most elevators were under utilised and hence more runs to the construction sites were required to transport the material than if the elevator was fully loaded. However,
with the Poka-Yoke device the elevator would move only if the loading doors were closed and then the
required load/raw material would be transported. This reduced the project lead-time.
This was not the case with all construction sites. Most of them did not have even performance indicators so the process was erratic.
Another Poka-Yoke Device
Apart from the elevator Poka-Yoke device, another commonly used Poka-Yoke device was for mortar
mixing. One of the most critical stages of mortar mixing is the stage when water is mixed. This not only
decides the quality of the mortar but also accounts for its strength. In most of the construction sites
studied, workers pour water to prepare mortar manually.
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Researchers observed that often-excess water was added to the mix thereby diminishing its quality.
Eventually, the mix would be thrown away and a new mix would be prepared. This resulted in wastage
of large volumes of raw material (cement/sand/gravel).
In countries where labour is cheap, construction companies often hire new and inexperienced employees. Does inexperience contribute to raw material wastage? Chances are fairly high!! Ironically, construction companies persist with such practices. This is primarily because they do not realise that they
are losing a lot of money and resources, because it goes unaccounted.
Installing Poka-Yoke devices can eliminate such problems. These devices prevent excess water into
the mortar mix that might eventually weaken the structure. This apart, they save a lot of money as raw
material wastage is reduced. Furthermore, the project lead-time is reduced.
Poka-Yoke/mistake proofing does not always require devices. Design too can help reduce the number
of defects. For instance, a window fabricating company in Brazil was getting complaints from its customers regarding its windows.
Investigation revealed that the sub contractor's company often forgot to put putty (A thick, mastic type
material, often made from calcium carbonate and a linseed oil, used for setting glass in wood frames
and filling cracks, holes, etc.). As a result, the glass pane fell off the wooden frame.
To avoid this problem the company decided to attach the window frames with a coating, which was to
be removed to fix the glass plane. Removing the coating revealed a layer of putty. Thus, the risk of the
glass pane falling off was reduced.
Conclusion
Manufacturers are now faced with the challenge of a 'cost-and-quality' competitive market. The quality
levels that they have to achieve are demanding and require stringent inspection systems. Companies
now believe that measuring parts on the plant floor improves product quality besides minimising inspection time. Organisations desirous of achieving excellent quality should aim at eliminating system
faults. Poka-Yoke is one such fail-safe tool/methodology that averts mistakes.
Useful links
Concept and application
1. John Grout's Poka-Yoke centre Provides step-by-step reading recommendations to help learn
mistake proofing better. Glossaries, everyday examples, a tutorial, and other useful resources are
provided in this document.
http://www.campbell.berry.edu/faculty/jgrout/pokayoke.shtml
2. Poka-Yoke - An explanation
This PDF document explains:
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
The various causes of defects

Elimination of defects

The applicable tools

The effectiveness of Poka-Yoke
http://www.pepsan.com/Poka-Yoke_tutorial.pdf
3. Poka-Yoke Mistake Proofing:
Poka-Yoke also called Mistake Proofing is a simple method to prevent or at least, detect and weed out
defects, as early as possible in the process. This document explains the above concept with some real
time examples.
http://www.isixsigma.com/library/content/c020128a.asp
4. Mistake Proofing - Work instructions
This document provides step-by-step instructions for mistake proofing an organisation's processes.
http://www.isixsigma.com/library/content/c020828a.asp
5. Mistake Proofing: An effective quality tool
This document explains

Mistake proofing in three simple steps.

Similarity and difference between FMEA and Poka-Yoke.

Examples of common mistake proofing methods.
http://rac.alionscience.com/pdf/MistakeProofing(PokaYoke).pdf
6. Poka Yoke for Environmental Quality Management
This Document explains:

What Poka Yoke is?

Poka Yoke devices and their characteristics

The three levels of Poka Yoke

Where Poka Yoke works and where it doesn't?
http://www.pojasek-associates.com/Reprints/PokaYoke.doc
7. Error Proofing (Poka Yoke): Common Causes and the steps for applying it in supply chain
This page explains how human errors and equipment failures can be minimised through error-proofing
techniques to improve supply chain performance.
http://www.iwvaluechain.com/Columns/columns.asp?ColumnId=846
Presentations
8. Poka Yoke: Presentation:
This presentation displays and explains how mistake proofing can be used towards achieving Zero
Defect quality. Besides explaining the concept it also presents evidences of the effectiveness of Mistake Proofing backing it up with some real time examples.
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http://www.campbell.berry.edu/faculty/jgrout/tqe.ppt
9. Presentation - Mistake Proofing
This page contains four good presentation covering the topics 5S, Lean/Kaizen, Mistake Proofing, and
Quick Changeover. While the sample presentations can be freely downloaded the full presentations
will have to be purchased.
http://www.landp.com.au/special/free.php
Training and consulting services
10. Poka-Yoke - Work shop
This page provides information regarding an interactive workshop that focuses on getting people to
recognize their critical role in driving down problems and defects in both the internal and external customer chains.
http://www.highperformancesolutions.ca/training_pokayoke.htm
11. PSB Corporation Training and Consulting Solutions:
PSB CORPORATION is an integrated solution provider assisting organisations accomplish business
excellence through people, products and processes. Located in China, Indonesia, Malaysia and Thailand PSB Corporation provides both training and consulting for various Quality tools including Poka
Yoke.
http://www.psbcorp.com/consulting/trgsolutions_prosysexcel_costreduct_poka.shtml
12. Mistake Proofing: Web based training
Outbound Train is the leading Internet distributor of interactive multimedia courseware for
employee training, specialising in the different courseware that also includes poka yoke. Over 100
Mistake-Proofing examples along with traditional Poka Yoke techniques and innovative
application tools and strategies are provided.
http://www.outboundtrain.com/html/reseng/mistakeproof.html
13. Profit Technologies:
Based in Riverside, California Profit technologies offers consulting and training for various quality
tools and techniques that includes Poka Yoke or Mistake proofing.
http://www.profitec.com/training.html
Books
1. The Basics of Mistake-Proofing by Michael R. Beauregard, Raymond J. Mikulak, Robin E. McDermott
Book Description
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The Basics of Mistake-Proofing will show you how to identify and use appropriate methods to create
safe products and processes. This book explains each type of mistake-proofing simply and clearly for
all levels of employees and it is especially useful for team, work cell, and production employees. The
book helps you to identify which type of mistake-proofing to use for a particular application. The book
also provides

A simple 5-step mistake-proofing process.

How to do mistake-proofing based on FMEAs.

Examples of each type of mistake-proofing and when to use
2. Zero Quality Control: Source Inspection and the Poka-Yoke System by Shigeo Shingo, Andrew P.
Dillon (Translator)
Book Description
Dr. Shingo reveals his unique defect prevention system, which combines source inspection and PokaYoke (mistake-proofing) devices that provide instant feedback on errors before they can become defects. The result: 100 percent inspection that eliminates the need for SQC and produces defect-free
products. Includes 112 examples, most costing under $100. Two-part video program also available;
3. Poka-Yoke: Improving Product Quality by Preventing Defects by Nikkan Kogyo Shimbun (Editor),
Factory Magazine (Editor), Nikkan Kogyo Shimbun Ltd, Nikkan Kogyo Shimbun, Ltd., Factory Magazine
Book Description
If your goal is 100 percent zero defects, this book is for you - a completely illustrated guide to PokaYoke (mistake-proofing) for supervisors and shop-floor workers. Many Poka-Yoke devices come from
line workers and are implemented with the help of engineering staff. The result is better product quality
- and greater participation of workers in improving your processes, products, and your company as a
whole.
4. Poka-Yoke: Mistake-Proofing Module by Geoffrey L. Mika
Book Description
Eliminate all errors and mistakes in the workplace. Better than Six-Sigma! Simple to learn and implement it works everywhere! . Sensei Mika used this workbook to conduct Poka-Yoke events in over
200 events. Now you can have the same workbook!
5. Mistake-Proofing for Operators: The Zqc System (Shopfloor Series) by Productivity Press Development Team (Creator), Productivity Development Team (Creator), Shingo
Book Description
ZQC is based on the principle that defects are prevented by controlling the performance of a process
so that it cannot produce defects, even when a machine or a human operator commits a mistake .
Instead of pointing fingers at people, ZQC finds ways to keep errors from turning into defects. This
book introduces front-line workers to the basic methodology of ZQC in a succinct, easy to read format
that covers all aspects of this important manufacturing improvement strategy.
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