Mistake Proofing

Also called: poka-yoke, fail–safing

Mistake proofing, or its Japanese equivalent poka-yoke (pronounced PO-ka yo-KAY), is the use of any automatic device or method that either makes it impossible for an error to occur or makes the error immediately obvious once it has occurred.

When to Use Mistake Proofing

• When a process step has been identified where human error can cause mistakes or defects to occur, especially in processes that rely on the worker’s attention, skill or experience.
• In a service process, where the customer can make an error which affects the output.
• At a hand-off step in a process, when output or (for service processes) the customer is transferred to another worker.
• When a minor error early in the process causes major problems later in the process.
• When the consequences of an error are expensive or dangerous.

Mistake–Proofing Procedure

1. Obtain or create a flowchart of the process. Review each step, thinking about where and when human errors are likely to occur.
2. For each potential error, work back through the process to find its source.
3. For each error, think of potential ways to make it impossible for the error to occur. Consider:
• Elimination—eliminating the step that causes the error.
• Replacement—replacing the step with an error-proof one.
• Facilitation—making the correct action far easier than the error.
4. If you cannot make it impossible for the error to occur, think of ways to detect the error and minimize its effects. Consider inspection method, setting function and regulatory function.
5. Choose the best mistake-proofing method or device for each error. Test it, then implement it. Three kinds of inspection methods provide rapid feedback:
• Successive inspection is done at the next step of the process by the next worker.
• Self-inspection means workers check their own work immediately after doing it.
• Source inspection checks, before the process step takes place, that conditions are correct. Often it’s automatic and keeps the process from proceeding until conditions are right.

Setting functions are the methods by which a process parameter or product attribute is inspected for errors:

• The contact or physical method checks a physical characteristic such as diameter or temperature, often using a sensor.
• The motion-step or sequencing method checks the process sequence to make sure steps are done in order.
• The fixed-value or grouping and counting method counts repetitions or parts or weighs an item to ensure completeness.
• A fourth setting function is sometimes added: information enhancement. This makes sure information is available and perceivable when and where required.

Regulatory functions are signals that alert the workers that an error has occurred:

• Warning functions are bells, buzzers, lights and other sensory signals. Consider using color-coding, shapes, symbols and distinctive sounds.
• Control functions prevent the process from proceeding until the error is corrected (if the error has already taken place) or conditions are correct (if the inspection was a source inspection and the error has not yet occurred).

Mistake–Proofing Example

The Parisian Experience restaurant wished to ensure high service quality through mistake-proofing. They reviewed the deployment chart (a detailed flowchart that shows who performs each step) of the seating process shown in Figure 1 and identified human errors on the part of restaurant staff or customers that could cause service problems.

Figure 1 Restaurant’s deployment chart

The first potential error occurs when customers enter. The maitre d’ might not notice a customer is waiting if the maitred’ is escorting other customers to their table, checking on table status or conferring with kitchen staff.

The mistake-proofing device is an electronic sensor on the entrance door. The sensor sends a signal to a vibrating pager on the maitre’s belt to ensure that the maitre d’ always knows when someone enters or leaves the restaurant. Other mistake-proofing methods replaced the process steps requiring the maitre d’ to leave the front door to seat customers.

A possible error on the customers’ part was identified at the step when diners are called from the lounge when their table is ready. They might miss the call if the lounge is noisy, if they are engrossed in conversation or if they are hard-of-hearing.

The mistake-proofing chosen by the team was to replace the step of the process in which the maitre d’ called the customer’s name over the loudspeaker. Instead, during the greeting step, the maitre d’ notes a unique visual identifier of one or more members of the party. When the table is ready, the table busser notifies the waiter, who comes to the maitre d’ and learns how to identify the customers. The waiter finds the customers in the lounge, escorts them to their table, gives them menus and takes additional drink orders.

Not only does this mistake-proofing method eliminate a customer-caused problem, it improves the restaurant ambiance by eliminating the annoying loudspeaker, keeps the maitre d’ at the front door to greet customers, creates a sense of exceptional service when the waiter “magically” knows the customers and eliminates additional waiting time at the handoff between maitre d’ and waiter.

Excerpted from Nancy R. Tague’s The Quality Toolbox, Second Edition, ASQ Quality Press, 2004, pages 351–356.