Mistake Proofing Overview

[Steven Bonacorsi]

Mistake proofing is a technique for eliminating errors. It is based
upon the premise that it is good to do something right the first time;
it is even better to make it impossible to do it wrong the first time.
The idea is to make it impossible to make a mistake. You may also hear
the term, Poka Yoke or Error Proofing applied to mistake proofing.

The objectives of this article are to provide participants with the
information so they can:


•Understand the difference between errors and defects;
•Understand how defects originate
•Recognize the elements of source inspection and its role in defect
elimination;
•Identify key mistake proofing devices;
•Show mistake proofing as a proactive tool, and;
•Show how mistake proofing fits into the Lean Six Sigma methodology.


Principles for Mistake Proofing

Mistake proofing is rooted in a deep respect for the intelligence of
workers. It is a principle that seeks to take over repetitive tasks or
actions thereby freeing a worker's time for creative and value-adding
activities. Its objective is zero defects.

There are several examples of mistake proofing in everyday life.
Consider automatic sinks and toilets in public rest rooms. They are
designed to turn on and off without having to think about them.
Automatic seat belts work the same way. All one needs to do is sit in
the car and turn on the key and a shoulder harness moves into place.
Mistake proofing focuses on eliminating the source of errors.

Errors vs. Defects

Errors are not the same as defects. Errors cause defects. Defects are
many times the result of errors.

Walk into any public restroom and you are likely to see toilets that
have not been flushed or sinks, with water running freely. In many
cases, the hot water has been turned off in the sinks to conserve the
costs of energy for heating the water. In these circumstances, what
are the defects and what are the errors?

The defects are the filthy toilets and the running water. These are
nonconformities in the process. They do not meet the predetermined
criteria that the public and management have established for a good
restroom. As such, they must be corrected. If not corrected, they will
cost management, both in terms of dollars and customers.

Hot water that runs unchecked involves a significant fuel expense.
Filthy toilets discourage the public from returning to the business.
What is management to do?

One solution is to get out of the business. Many public places take
this approach. Their restrooms are closed to the public. A modified
version of this solution is to limit services to limit expenses. This
is the approach taken when management turns off hot water. The hope is
that the customer does not miss it. Another solution is to inspect for
defects frequently and correct them when found. Yet this solution is
only effective if its cost is less than or equal to the cost of the
defect. Most often it is a stop-gap measure that provides fewer
defects but at greater costs. Mistake proofing offers a third
solution.

The question is, can the error that causes the defect be minimized?
Can it be eliminated? The error in our public rest room example is
human forgetfulness. People forget to operate the equipment. So, we
build in an automated solution that takes care of this operation for
them. They can no longer make the error; the defect is removed.

There are five general reasons why errors occur:


•Procedures are incorrect or non-existent. They do not fit the task,
the environment, the equipment, the materials, the measurement, or the
labor resources.
•There is excessive variation in the process
•There is excessive variation in the raw materials
•Measuring devices are inaccurate
•Human error


The last item on our list, human error, can be further broken down
into ten categories:


•Forgetfulness, nor not concentrating;
•Misunderstanding, or jumping to conclusions without having all the
information;
•Identification, or viewing the situation incorrectly (perhaps it is
too far away);
•Training, or lack of it;
•Willful errors where the human ignores the rules;
•Inadvertent errors involving distraction or fatigue;
•Slowness, or delays in judgment;
•Lack of standards;
•Surprises when something does not go as planned, and;
•Intentional errors such as sabotage.


This list should give some food for thought. There are many things
that contribute to human error. Among them are adjustments, multiple
tasking, infrequent tasks, lack of standards, and repetition that is
too rapid for the individual.

Such things can be controlled. Indeed, a great deal of planning and
expense is involved in trying to do so. Yet errors still happen. A
traditional approach to manufacturing and service industries is to
think they are unavoidable. People are only human. Everything varies,
and you can't plan for all contingencies. The traditionalist would say
that everything must be inspected to control the inevitable defect.
(Remember that defect is the result of error.)

The Lean Six Sigma view, however, differs. Many errors can be
eliminated, not all of them, to be sure, but many. If many can be
eliminated, and many more can be reduced, and if the focus of the
efforts is in the Vital Few rather than the Trivial Many, then the
need for inspection can be reduced or eliminated. Inspection for
defects does not capture all of them. A Lean Six Sigma approach makes
the User the inspector, and it controls the errors in the processes to
assure that the user is satisfied with their inspections. Inspection
happens in a Lean Six Sigma process; it is an inspector for error.

Source Inspection

The chart below illustrates the traditional inspection process that
focuses on finding the defect. Here the inspector stands at the end of
the line and feeds back information to its various positions.

Mistake proofing provides immediate feedback at each step, prior to
the creation of a defect. Detection is finding an error after it has
occurred. Indeed, action can be taken before errors occur. Immediate
feedback and immediate proximity to the potential error allows for
better Prediction and Prevention.

Mistake Proofing On the Job

Shutdown: Effective in both prediction and detection. An example of a
shutdown device is a camera that will not function when there is not
enough light to take a picture. The meter predicts the picture will
not take, and it shuts down. Some clothes dryers shut down when they
detect an overheating situation.

Control: A control device can make pending errors impossible.
Erroneous items cannot be used in the process. When gas stations
introduced unleaded gasoline, the nozzle on the leaded pump was
designed to be too big to fit into an unleaded tank, thereby
preventing error. A fruit orchard assures that only the biggest apples
pass on to its customers by passing the apples through a sizer. Apples
that are too small fall through and are sent to a discount outlet.

Warning: A warning device predicts when something is about to go
wrong. It can also sound immediately when something does go wrong.
Seat belt buzzers are warning devices used for prediction. Smoke
detectors detect a hazardous situation.

Contact Methods: Contact with a part highlights errors. Electrical
outlets have been mistake proofed to assure proper polarity. It is
impossible to put a plug in an outlet incorrectly.

Fixed Value Methods: Errors are detected through counting. Many boxes
should be shipped with four different hazardous material warning
labels. When labels were on separate rolls, errors in shipping often
occurred. By placing all labels on a common roll and aligning them for
easy reference, the worker easily knew when a label was missed.

Motion Step Methods: Detect errors by motion, or the lack of it. Seven
screws in various sizes were inserted in the final assembly of a CD-
ROM drive. Often a screw would be forgotten. The seven Different
screws were put into bins with photo-electric switches. When a screw
is removed, the beam is broken. The part cannot move on to the next
operation until the beam is broken on all seven bins.

As you begin to recognize the techniques and methods that apply to
mistake proofing, it is easy to see how the principal has been
applied. Limit switches are commonly applied to assure that machines
are not operated beyond their design capacity or that they are
operated in a safe manner. Guide pins are routinely used to assure
that jigs and molds go together in the correct configuration. At the
end of this module are eight mistake proofing challenges. How can the
problems be solved?

When You Can't Mistake Proof

Some strategies can be employed to minimize mistakes, or make them
easier to detect. Consider the following:


•Colors and color coding. Credit card receipts are almost always
designed so the customer gets the yellow copy and the merchant gets
the white copy.
•Use of shapes. Stop signs are always octagonal.
•Auto-detection. Word processors come with spell checkers. These
reduce errors considerably.
•Make it easy to do it right. Use checklists. Have effective data
collection forms that follow the process. Design work flows with fewer
hand-offs.


5 Steps to Mistake Proofing: There are five steps to mistake proofing:

1. Identify problems. Look at customer returns, defective parts
analyses, and error reports. Do a FMEA.

2. Prioritize problems. Look at problem frequency and cost. Identify
wasted materials, rework time, detection time, and detection cost.
Consider inspection costs.

3. Seek out the root cause. Drive down until you truly understand the
source of the error. Correct mistakes at their source.

4. Create solutions. Make it impossible to do it wrong. Perform a cost
benefit analysis to see how long the investment in the solution will
take to repay itself. Be creative in solution generation.

5. Measure the results. Have errors been eliminated and what is the
impact? There is no doubt that mistake proofing offers several
advantages when it is designed into the product. No formal training
programs are required to implement it or to use the devices. It
eliminates many inspection operations, and it relieves operators from
repetitive tasks that take away from their creativity and value added
activities. It results in defect-free work. It provides immediate
action when problems do arise.

Mistake Proofing Challenges

Look at the following situations. What mistake proofing methods or
techniques might you consider applying?


1.ATM customers complain that they never know the right way to insert
their ATM card, and it usually takes a couple of tries to get it right
2.A local bottling company wants to make sure that all bottles are
filled with exactly the right amount of soda.
3.A large pharmaceutical company is trying to develop a way to assure
elderly people living alone take their prescription medicine at the
right time and in the correct dosage.
4.A manufacturer of home power tools wants to make sure that
customers are wearing eye and hand protection before using their
equipment.
5.A consumer electronic company has had several customer complaints
lately that their instructions were missing from their product.
6.Nine different tools are needed to change a jig and die. Often, in
the middle of changeover, the set-up operator realizes a tool is
missing and has to stop to look for the tool.
7.An easy-to-assemble furniture manufacturer has received several
complaints about holes not being tapped for all screws. Because the
furniture is made from hardwood, the customers have to drill the holes
themselves.
8.A contact lens solution manufacturer received complaints from
distributors that some customers found empty boxes with their
shipments. The company wants to make sure no more empty boxes are
shipped.



Steven Bonacorsi is a Certified Lean Six Sigma Master Black Belt
instructor and coach. Steven Bonacorsi has trained hundreds of Master
Black Belts, Black Belts, Green Belts, and Project Sponsors and
Executive Leaders in Lean Six Sigma DMAIC and Design for Lean Six
Sigma process improvement methodologies. Bought to you by the Process
Excellence Network the world leader in Business Process Management
(BPM)

Author for the Process Excellence Network (PEX Network / IQPC)

Process Excellence Network
Steven Bonacorsi, President of International Standard for Lean Six
Sigma(ISLSS)
Certified Lean Six Sigma Master Black Belt
47 Seasons Lane
Londonderry, NH 03053
Phone: +(1) (603) 401-7047
E-mail: sbona...@islss.com
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