Poka Yoke

Mistake Proofing 

Error Proofing

What is Poka Yoke?

Poka Yoke is defined as the process to change the method or equipment to ensure that a particular error cannot happen. It is a method to prevent mistakes that cause defects.

Poka Yoke is known by many names such as  Mistake Proofing  or Error Proofing. “Poka Yoke” is a Japanese term coined by engineer Shigeo Shingo. Poka-Yoke comes from the Japanese words for “inadvertent error” and “avoidance.” Literally translated, the term means “fool-proofing,” but Dr. Shingo was concerned the term would offend many workers, so the term was changed to “mistake-proofing.” Also known as error-proofing, or countermeasure, the terms all describe the same concept: to ensure that a defect cannot occur. Mistake-proofing is the ongoing pursuit of perfection and is critical to sustaining zero defects.

Evidence of mistake-proofing is everywhere. Consider these examples: In most vehicles in the marketplace, you’ll notice that your gas cap is attached to the car by a tether. That means you can’t just drive away from the filling station and leave your gas cap behind.

Some homes are built with a keypad to let you into the house. You can’t lock yourself out of the house this way. Clothing dryers stop when you open the door. This prevents individual injury; otherwise, you can get your hand/clothes caught in the spinning drum. Online forms very commonly have pre-populated drop-down lists so we can avoid certain typing mistakes.

Relationship Between Errors and Defects

To understand the need for mistake-proofing, you must first understand the relationship between a defect and an error condition. An error condition is the cause of a defect. Consider this example: It’s morning, and you are ironing your shirt for work. The phone rings in another room, and you quickly run to pick it up. After your conversation, you remember that you were in the middle of ironing your shirt, so you return to the ironing board to find a ruined shirt.

In this scenario, the burnt shirt is your defect, and forgetting to shut off the iron is the error condition, or cause. Mistake-proofing seeks to eliminate the error condition, thus removing the potential for any defect to occur.

Customers demand defect-free products and processes. You might argue that a defect-free product or service is impossible to achieve, but the goal of any performance excellence initiative is to reduce the potential for error. Mistake-proofing is the best way we can eliminate the possibility that an error will occur.

The 1-10-100 rule is a good illustration of why mistake-proofing is so important. This rule implies that if it will take only one hour to fix a problem as soon as it occurs, it will take 10 hours downstream, or 100 hours to fix after it reaches the customer. By mistake-proofing the process so errors are caught before they happen, we spend less time and money on inspection techniques, scrap, and rework.

Mistake-proofing can be applied to every product. The challenge for any performance excellence practitioner is to apply mistake-proofing to a process. How do you prevent defects from being passed from one process step to another? The ideal is to eliminate the possibility of a mistake occurring. This is pure mistake-proofing. But in reality, you strive for the perfect solution while you implement methods that reduce the likelihood of creating a mistake. An example is Microsoft Word’s “spell-checker” software. It has reduced the likelihood of spelling errors and thus is a good improvement, but it has not eliminated all errors.

Poka Yoke Method

There is a set of steps you can follow to implement mistake-proofing in your process, and these steps can be implemented reactively in response to the observation of defects, or proactively when high-risk areas are identified in the process.

Step 1: Describe the defect in detail. Examine the history of the defect in terms of defect rate over time.

Step 2: Identify the location of the defect within the process flow. Where was the defect created, and where was it discovered?

Step 3: Determine the root cause. Conduct a cause and effects analysis to assess the root cause. Consider the types of errors that may create the defect and the condition that may trigger the error.

Step 4: Review the current standard operating procedure, or SOP. Document each step in the operation where the error and defect occurs.

Step 5: Identify any deviation from standards. Observe the actual process and identify areas where methods being applied deviate from the standard operating procedure. These deviations will point to areas where procedural improvements are needed.

Step 6: Identify the type of error-proofing device type that will effectively eliminate the defect. Focus on prevention first and detection second.

Step 7: Create a device or method, and test for effectiveness. Make modifications until the device proves effective in preventing the defect.

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