| Mistake proofing is a technique for eliminating errors. It | | | | is finding an error after it has occurred. |
| is based upon the premise that it is good to do | | | | Indeed, action can be taken before errors occur. |
| something right the first time; it is even better to make | | | | Immediate feedback and immediate proximity to the |
| it impossible to do it wrong the first time. The idea is to | | | | potential error allows for better Prediction and |
| make it impossible to make a mistake. You may also | | | | Prevention. |
| hear the term, Poka-Yoke or Error Proofing applied to | | | | Mistake Proofing On the Job |
| mistake proofing. | | | | Shutdown: Effective in both prediction and detection. |
| The objectives of this article are to provide participants | | | | An example of a shutdown device is a camera that |
| with the information so they can: | | | | will not function when there is not enough light to take |
| - Understand the difference between errors and | | | | a picture. The meter predicts the picture will not take, |
| defects; | | | | and it shuts down. Some clothes dryers shut down |
| - Understand how defects originate | | | | when they detect an overheating situation. |
| - Recognize the elements of source inspection and its | | | | Control: A control device can make pending errors |
| role in defect elimination; | | | | impossible. Erroneous items cannot be used in the |
| - Identify key mistake proofing devices; | | | | process. When gas stations introduced unleaded |
| - Show mistake proofing as a proactive tool, and; | | | | gasoline, the nozzle on the leaded pump was designed |
| - Show how mistake proofing fits into the Lean Six | | | | to be too big to fit into an unleaded tank, thereby |
| Sigma methodology. | | | | preventing error. A fruit orchard assures that only the |
| Principles for Mistake Proofing | | | | biggest apples pass on to its customers by passing |
| Mistake proofing is rooted in a deep respect for the | | | | the apples through a sizer. Apples that are too small |
| intelligence of workers. It is a principle that seeks to | | | | fall through and are sent to a discount outlet. |
| take over repetitive tasks or actions thereby freeing a | | | | Warning: A warning device predicts when something is |
| worker's time for creative and value-adding activities. | | | | about to go wrong. It can also sound immediately |
| Its objective is zero defects. | | | | when something does go wrong. Seat belt buzzers |
| There are several examples of mistake proofing in | | | | are warning devices used for prediction. Smoke |
| everyday life. Consider automatic sinks and toilets in | | | | detectors detect a hazardous situation. |
| public rest rooms. They are designed to turn on and | | | | Contact Methods: Contact with a part highlights errors. |
| off without having to think about them. Automatic seat | | | | Electrical outlets have been mistake proofed to assure |
| belts work the same way. All one needs to do is sit in | | | | proper polarity. It is impossible to put a plug in an outlet |
| the car and turn on the key and a shoulder harness | | | | incorrectly. |
| moves into place. Mistake proofing focuses on | | | | Fixed Value Methods: Errors are detected through |
| eliminating the source of errors. | | | | counting. Many boxes should be shipped with four |
| Errors vs. Defects | | | | different hazardous material warning labels. When |
| Errors are not the same as defects. Errors cause | | | | labels were on separate rolls, errors in shipping often |
| defects. Defects are many times the result of errors. | | | | occurred. By placing all labels on a common roll and |
| Walk into any public restroom and you are likely to see | | | | aligning them for easy reference, the worker easily |
| toilets that have not been flushed or sinks, with water | | | | knew when a label was missed. |
| running freely. In many cases, the hot water has been | | | | Motion Step Methods: Detect errors by motion, or the |
| turned off in the sinks to conserve the costs of | | | | lack of it. Seven screws in various sizes were inserted |
| energy for heating the water. In these circumstances, | | | | in the final assembly of a CD-ROM drive. Often a |
| what are the defects and what are the errors? | | | | screw would be forgotten. The seven Different |
| The defects are the filthy toilets and the running water. | | | | screws were put into bins with photo-electric switches. |
| These are non-conformities in the process. They do | | | | When a screw is removed, the beam is broken. The |
| not meet the pre-determined criteria that the public and | | | | part cannot move on to the next operation until the |
| management have established for a good restroom. | | | | beam is broken on all seven bins. |
| As such, they must be corrected. If not corrected, they | | | | As you begin to recognize the techniques and |
| will cost management, both in terms of dollars and | | | | methods that apply to mistake proofing, it is easy to |
| customers. | | | | see how the principal has been applied. Limit switches |
| Hot water that runs unchecked involves a significant | | | | are commonly applied to assure that machines are not |
| fuel expense. Filthy toilets discourage the public from | | | | operated beyond their design capacity or that they are |
| returning to the business. | | | | operated in a safe manner. Guide pins are routinely |
| What is management to do? | | | | used to assure that jigs and molds go together in the |
| One solution is to get out of the business. Many public | | | | correct configuration. At the end of this module are |
| places take this approach. Their restrooms are closed | | | | eight mistake proofing challenges. How can the |
| to the public. A modified version of this solution is to | | | | problems be solved? |
| limit services to limit expenses. This is the approach | | | | When You Can't Mistake Proof |
| taken when management turns off hot water. The | | | | Some strategies can be employed to minimize |
| hope is that the customer does not miss it. Another | | | | mistakes, or make them easier to detect. Consider the |
| solution is to inspect for defects frequently and correct | | | | following: |
| them when found. Yet this solution is only effective if | | | | - Colors and color coding. Credit card receipts are |
| its cost is less than or equal to the cost of the defect. | | | | almost always designed so the customer gets the |
| Most often it is a stop gap measure that provides | | | | yellow copy and the merchant gets the white copy. |
| fewer defects but at greater costs. Mistake proofing | | | | - Use of shapes. Stop signs are always octagonal. |
| offers a third solution. | | | | - Auto-detection. Word processors come with spell |
| The question is, can the error that causes the defect | | | | checkers. These reduce errors considerably. |
| be minimized? Can it be eliminated? | | | | - Make it easy to do it right. Use checklists. Have |
| The error in our public rest room example is human | | | | effective data collection forms that follow the process. |
| forgetfulness. People forget to operate the equipment. | | | | Design work flows with fewer hand-offs. |
| So, we build in an automated solution that takes care | | | | 5 Steps to Mistake Proofing: |
| of this operation for them. They can no longer make | | | | There are five steps to mistake proofing: |
| the error; the defect is removed. | | | | 1. Identify problems. Look at customer returns, |
| There are five general reasons why errors occur: | | | | defective parts analyses, and error reports. Do a |
| - Procedures are incorrect or non-existent. They do | | | | FMEA. |
| not fit the task, the environment, the equipment, the | | | | 2. Prioritize problems. Look at problem frequency and |
| materials, the measurement, or the labor resources. | | | | cost. Identify wasted materials, rework time, detection |
| - There is excessive variation in the process | | | | time, and detection cost. Consider inspection costs. |
| - There is excessive variation in the raw materials | | | | 3. Seek out the root cause. Drive down until you truly |
| - Measuring devices are inaccurate | | | | understand the source of the error. Correct mistakes |
| - Human error | | | | at their source. |
| The last item on our list, human error, can be further | | | | 4. Create solutions. Make it impossible to do it wrong. |
| broken down into ten categories: | | | | Perform a cost benefit analysis to see how long the |
| - Forgetfulness, nor not concentrating; | | | | investment in the solution will take to repay itself. Be |
| - Misunderstanding, or jumping to conclusions without | | | | creative in solution generation. |
| having all the information; | | | | 5. Measure the results. Have errors been eliminated |
| - Identification, or viewing the situation incorrectly | | | | and what is the impact? |
| (perhaps it is too far away); | | | | There is no doubt that mistake proofing offers several |
| - Training, or lack of it; | | | | advantages when it is designed into the product. No |
| - Willful errors where the human ignores the rules; | | | | formal training programs are required to implement it or |
| - Inadvertent errors involving distraction or fatigue; | | | | to use the devices. It eliminates many inspection |
| - Slowness, or delays in judgment; | | | | operations, and it relieves operators from repetitive |
| - Lack of standards; | | | | tasks that take away from their creativity and value |
| - Surprises when something does not go as planned, | | | | added activities. It results in defect-free work. It |
| and; | | | | provides immediate action when problems do arise. |
| - Intentional errors such as sabotage. | | | | Mistake Proofing Challenges |
| This list should give some food for thought. There are | | | | Look at the following situations. What mistake proofing |
| many things that contribute to human error. Among | | | | methods or techniques might you consider applying? |
| them are adjustments, multiple tasking, infrequent tasks, | | | | |
| lack of standards, and repetition that is too rapid for | | | | 1. ATM customers complain that they never know the |
| the individual. | | | | right way to insert their ATM card, and it usually takes |
| Such things can be controlled. Indeed, a great deal of | | | | a couple of tries to get it right |
| planning and expense is involved in trying to do so. Yet | | | | 2. A local bottling company wants to make sure that all |
| errors still happen. A traditional approach to | | | | bottles are filled with exactly the right amount of soda. |
| manufacturing and service industries is to think they | | | | 3. A large pharmaceutical company is trying to develop |
| are unavoidable. People are only human. Everything | | | | a way to assure elderly people living alone take their |
| varies, and you can't plan for all contingencies. The | | | | prescription medicine at the right time and in the |
| traditionalist would say that everything must be | | | | correct dosage. |
| inspected to control the inevitable defect. (Remember | | | | 4. A manufacturer of home power tools wants to |
| that defect is the result of error.) | | | | make sure that customers are wearing eye and hand |
| The Lean Six Sigma view, however, differs. Many | | | | protection before using their equipment. |
| errors can be eliminated, not all of them, to be sure, but | | | | 5. A consumer electronic company has had several |
| many. If many can be eliminated, and many more can | | | | customer complaints lately that their instructions were |
| be reduced, and if the focus of the efforts is in the | | | | missing from their product. |
| Vital Few rather than the Trivial Many, then the need | | | | 6. Nine different tools are needed to change a jig and |
| for inspection can be reduced or eliminated. Inspection | | | | die. Often, in the middle of changeover, the set-up |
| for defects does not capture all of them. A Lean Six | | | | operator realizes a tool is missing and has to stop to |
| Sigma approach makes the User the inspector, and it | | | | look for the tool. |
| controls the errors in the processes to assure that the | | | | 7. An easy-to-assemble furniture manufacturer has |
| user is satisfied with their inspections. Inspection | | | | received several complaints about holes not being |
| happens in a Lean Six Sigma process; it is an inspector | | | | tapped for all screws. Because the furniture is made |
| for error. | | | | from hardwood, the customers have to drill the holes |
| Source Inspection | | | | themselves. |
| The chart below illustrates the traditional inspection | | | | 8. A contact lens solution manufacturer received |
| process that focuses on finding the defect. Here the | | | | complaints from distributors that some customers |
| inspector stands at the end of the line and feeds back | | | | found empty boxes with their shipments. The |
| information to its various positions. | | | | company wants to make sure no more empty boxes |
| Mistake proofing provides immediate feedback at | | | | are shipped. |
| each step, prior to the creation of a defect. Detection | | | | |