Mistake Proofing

Mistake Proofing Overview
				Shutdown: Effective in both prediction and detection.
Mistake proofing is a technique for eliminating errors. It				An example of a shutdown device is a camera that
is based upon the premise that it is good to do				will not function when there is not enough light to take
something right the first time; it is even better to make				a picture. The meter predicts the picture will not take,
it impossible to do it wrong the first time. The idea is to				and it shuts down. Some clothes dryers shut down
make it impossible to make a mistake. You may also				when they detect an overheating situation.
hear the term, Poka-Yoke or Error Proofing applied to
mistake proofing.				Control: A control device can make pending errors
				impossible. Erroneous items cannot be used in the
The objectives of this article are to provide participants				process. When gas stations introduced unleaded
with the information so they can:				gasoline, the nozzle on the leaded pump was designed
- Understand the difference between errors and				to be too big to fit into an unleaded tank, thereby
defects;				preventing error. A fruit orchard assures that only the
- Understand how defects originate				biggest apples pass on to its customers by passing
- Recognize the elements of source inspection and its				the apples through a sizer. Apples that are too small
role in defect elimination;				fall through and are sent to a discount outlet.
- Identify key mistake proofing devices;
- Show mistake proofing as a proactive tool, and;				Warning: A warning device predicts when something is
- Show how mistake proofing fits into the Lean Six				about to go wrong. It can also sound immediately
Sigma methodology.				when something does go wrong. Seat belt buzzers
Principles for Mistake Proofing				are warning devices used for prediction. Smoke
				detectors detect a hazardous situation.
Mistake proofing is rooted in a deep respect for the
intelligence of workers. It is a principle that seeks to				Contact Methods: Contact with a part highlights errors.
take over repetitive tasks or actions thereby freeing a				Electrical outlets have been mistake proofed to assure
worker’s time for creative and value-adding				proper polarity. It is impossible to put a plug in an outlet
activities. Its objective is zero defects.				incorrectly.

There are several examples of mistake proofing in				Fixed Value Methods: Errors are detected through
everyday life. Consider automatic sinks and toilets in				counting. Many boxes should be shipped with four
public rest rooms. They are designed to turn on and				different hazardous material warning labels. When
off without having to think about them. Automatic seat				labels were on separate rolls, errors in shipping often
belts work the same way. All one needs to do is sit in				occurred. By placing all labels on a common roll and
the car and turn on the key and a shoulder harness				aligning them for easy reference, the worker easily
moves into place. Mistake proofing focuses on				knew when a label was missed.
eliminating the source of errors.
				Motion Step Methods: Detect errors by motion, or the
Errors vs. Defects				lack of it. Seven screws in various sizes were inserted
				in the final assembly of a CD-ROM drive. Often a
Errors are not the same as defects. Errors cause				screw would be forgotten. The seven Different
defects. Defects are many times the result of errors.				screws were put into bins with photo-electric switches.
				When a screw is removed, the beam is broken. The
Walk into any public restroom and you are likely to see				part cannot move on to the next operation until the
toilets that have not been flushed or sinks, with water				beam is broken on all seven bins.
running freely. In many cases, the hot water has been
turned off in the sinks to conserve the costs of				As you begin to recognize the techniques and
energy for heating the water. In these circumstances,				methods that apply to mistake proofing, it is easy to
what are the defects and what are the errors?				see how the principal has been applied. Limit switches
				are commonly applied to assure that machines are not
The defects are the filthy toilets and the running water.				operated beyond their design capacity or that they are
These are non-conformities in the process. They do				operated in a safe manner. Guide pins are routinely
not meet the pre-determined criteria that the public and				used to assure that jigs and molds go together in the
management have established for a good restroom.				correct configuration. At the end of this module are
As such, they must be corrected. If not corrected, they				eight mistake proofing challenges. How can the
will cost management, both in terms of dollars and				problems be solved?
customers.
				When You Can’t Mistake Proof
Hot water that runs unchecked involves a significant
fuel expense. Filthy toilets discourage the public from				Some strategies can be employed to minimize
returning to the business.				mistakes, or make them easier to detect. Consider the
				following:
What is management to do?				- Colors and color coding. Credit card receipts are
				almost always designed so the customer gets the
One solution is to get out of the business. Many public				yellow copy and the merchant gets the white copy.
places take this approach. Their restrooms are closed				- Use of shapes. Stop signs are always octagonal.
to the public. A modified version of this solution is to				- Auto-detection. Word processors come with spell
limit services to limit expenses. This is the approach				checkers. These reduce errors considerably.
taken when management turns off hot water. The				- Make it easy to do it right. Use checklists. Have
hope is that the customer does not miss it. Another				effective data collection forms that follow the process.
solution is to inspect for defects frequently and correct				Design work flows with fewer hand-offs.
them when found. Yet this solution is only effective if
its cost is less than or equal to the cost of the defect.				5 Steps to Mistake Proofing
Most often it is a stop gap measure that provides
fewer defects but at greater costs. Mistake proofing				There are five steps to mistake proofing:
offers a third solution.				1. Identify problems. Look at customer returns,
				defective parts analyses, and error reports. Do a
The question is, can the error that causes the defect				FMEA.
be minimized? Can it be eliminated?
				2. Prioritize problems. Look at problem frequency and
The error in our public rest room example is human				cost. Identify wasted materials, rework time, detection
forgetfulness. People forget to operate the equipment.				time, and detection cost. Consider inspection costs.
So, we build in an automated solution that takes care
of this operation for them. They can no longer make				3. Seek out the root cause. Drive down until you truly
the error; the defect is removed.				understand the source of the error. Correct mistakes
				at their source.
There are five general reasons why errors occur:
- Procedures are incorrect or non-existent. They do				4. Create solutions. Make it impossible to do it wrong.
not fit the task, the environment, the equipment, the				Perform a cost benefit analysis to see how long the
materials, the measurement, or the labor resources.				investment in the solution will take to repay itself. Be
- There is excessive variation in the process				creative in solution generation.
- There is excessive variation in the raw materials
- Measuring devices are inaccurate				5. Measure the results. Have errors been eliminated
- Human error				and what is the impact?
				There is no doubt that mistake proofing offers several
The last item on our list, human error, can be further				advantages when it is designed into the product. No
broken down into ten categories:				formal training programs are required to implement it or
- Forgetfulness, nor not concentrating;				to use the devices. It eliminates many inspection
- Misunderstanding, or jumping to conclusions without				operations, and it relieves operators from repetitive
having all the information;				tasks that take away from their creativity and value
- Identification, or viewing the situation incorrectly				added activities. It results in defect-free work. It
(perhaps it is too far away);				provides immediate action when problems do arise.
- Training, or lack of it;
- Willful errors where the human ignores the rules;				Mistake Proofing Challenges
- Inadvertent errors involving distraction or fatigue;
- Slowness, or delays in judgment;				Look at the following situations. What mistake proofing
- Lack of standards;				methods or techniques might you consider applying?
- Surprises when something does not go as planned,				1. ATM customers complain that they never know the
and;				right way to insert their ATM card, and it usually takes
- Intentional errors such as sabotage.				a couple of tries to get it right.

This list should give some food for thought. There are				1. A local bottling company wants to make sure that all
many things that contribute to human error. Among				bottles are filled with exactly the right amount of soda.
them are adjustments, multiple tasking, infrequent tasks,
lack of standards, and repetition that is too rapid for				1. A large pharmaceutical company is trying to develop
the individual.				a way to assure elderly people living alone take their
				prescription medicine at the right time and in the
Such things can be controlled. Indeed, a great deal of				correct dosage.
planning and expense is involved in trying to do so. Yet
errors still happen. A traditional approach to				1. A manufacturer of home power tools wants to
manufacturing and service industries is to think they				make sure that customers are wearing eye and hand
are unavoidable. People are only human. Everything				protection before using their equipment.
varies, and you can’t plan for all contingencies. The
traditionalist would say that everything must be				1. A consumer electronic company has had several
inspected to control the inevitable defect. (Remember				customer complaints lately that their instructions were
that defect is the result of error.)				missing from their product.

The Lean Six Sigma view, however, differs. Many				1. Nine different tools are needed to change a jig and
errors can be eliminated, not all of them, to be sure, but				die. Often, in the middle of changeover, the set-up
many. If many can be eliminated, and many more can				operator realizes a tool is missing and has to stop to
be reduced, and if the focus of the efforts is in the				look for the tool.
Vital Few rather than the Trivial Many, then the need
for inspection can be reduced or eliminated. Inspection				1. An easy-to-assemble furniture manufacturer has
for defects does not capture all of them. A Lean Six				received several complaints about holes not being
Sigma approach makes the User the inspector, and it				tapped for all screws. Because the furniture is made
controls the errors in the processes to assure that the				from hardwood, the customers have to drill the holes
user is satisfied with their inspections. Inspection				themselves.
happens in a Lean Six Sigma process; it is an inspector
for error.				1. A contact lens solution manufacturer received
				complaints from distributors that some customers
Source Inspection				found empty boxes with their shipments. The
				company wants to make sure no more empty boxes
The chart below illustrates the traditional inspection				are shipped.
process that focuses on finding the defect. Here the
inspector stands at the end of the line and feeds back				Steven Bonacorsi is a Senior Master Black Belt
information to its various positions.				instructor and coach. Steven Bonacorsi has trained
				hundreds of Master Black Belts, Black Belts, Green
Mistake proofing provides immediate feedback at				Belts, and Project Sponsors and Executive Leaders in
each step, prior to the creation of a defect. Detection				Lean Six Sigma DMAIC and Design for Lean Six
is finding an error after it has occurred.				Sigma process improvement methodologies.

Indeed, action can be taken before errors occur.				The AIT Group, Inc.
Immediate feedback and immediate proximity to the				Steven Bonacorsi, Vice President
potential error allows for better Prediction and				Lean Six Sigma Master Black Belt
Prevention.				3135 South Price Road, Suite 115
				Chandler, AZ 85248-3549
Mistake Proofing On the Job				Phone: +(1) 888.826.

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Mistake Proofing