Poka-Yoke & Error-proofing

Poka-yoke: error-proofing
Poka-yoke: error-proofing

Poka-yoke: error-proofing. Photo by Bill Abbot

Poka-yoke is the act of error proofing a process through great design. Defining requirements or ensuring continuous flow are good things but not strictly part of error-proofing.

A great example for anyone driving in the US is the different shapes and sizes of the gas pump nozzles. I can remember one bleary-eyed road trip in college where I was half awake when I tried to fill my 1986 Buick Regal with diesel instead of regular! Fortunately for me, poka-yoke has been used in the design of the gas nozzles; diesel gas is a triangle shape and thus impossible to fit in the cylindrical gas receiver my car had! That was great error proofing.

History Poka- Yoke

Coined in 1960s in Japan, Poka Yoke means “Fool- Proofing”, is a tool used to avoid inadvertent errors. The main premise of the tool is to ensure that proper conditions are placed for every process step such that there is no room for the occurrence of defects while execution. In other cases, it serves as a detector and helps to eliminate the defects as early as possible.

Poka Yoke is a vital tool in lean manufacturing since it helps the people and work processes work in right fashion in its very first time. It helps to eliminate defects out of the processes and products and improves the quality and reliability. It is worthy to note that it does mitigate both the mechanical as well as human errors from the work processes and products.

When to use Poke- Yoke?

The ideal time to use the technique of Poke- Yoke is when a process or product is likely to have error or may go wrong. The technique of Poke Yoke can easily be implemented in both the manufacturing as well as service industries. Some of the errors that are captured by Poke- Yoke are when the process operations are sub- standard or not operated at their standard operating procedure (processing errors), when the machines are not adjusted properly or tool used are incorrect (setup errors), when some part/s are missed at the assembly line (missing parts errors), when the parts actually used is not correct (improper parts error), when the specifications for the operation of the process or product is not correctly met (operations errors) and finally when the machines or parts are not adjusted, measured properly or in case of improper dimensions of the supplies or parts used (measurement errors).

How to use Poke- Yoke?

There are a number of steps that one can follow to implement Poke- Yoke:

Step 1: The process or operation that needs improvement needs to be identified using Pareto chart and analysis.

Step 2: Assess and understand the defects in the process that may lead to failure. At this stage, the 5- whys should be thoroughly analyzed.

Step 3: Decision about the strategy to be used for effective Poka- Yoke. Some of the strategies include

  • Shut out strategy that is, prevention strategy that the error is not committed
  • Attention strategy that is, pinpointing the defect when it is being committed
  • Comprehensive approaches

Step 4: Determine the appropriateness of the strategy being decided. This can be done by using a contact, that is any physical attribution for detection of error; constant number, in case the trigger behind the error is the absence of certain number of actions; and sequence method, that is to check and tally using a checklist that all the steps in the process are undertaken appropriately and effectively.

Step 5: Run a trial test to ensure that the method chosen is appropriate and effective.

Step 6: Once ensured that the method chosen is good, train the operators for the same, review the performance and measure the success to verify and validate the Poka- Yoke strategy.


ASQ Six Sigma Black Belt Certification Poka-yoke Questions:

Question: A company has installed a system that prevents orders with incorrect information from being forwarded to production scheduling. This is an example of which of the following lean tools? (Taken from ASQ sample Black Belt exam.)

(A) Standard work
(B) Kanban
(C) Poka-yoke
(D) Visual factory

Answer: (c) Poka-yoke is the answer because that is the concept of preventing errors through design. Making it impossible for users to submit bad information is a great example. Kanban is a system for increasing flow and visual factory is a way to efficiently transmit information.

Question: Poka-yoke is best defined as: (Taken from ASQ sample Black Belt exam.)

(A) improving machine efficiency
(B) reducing field failures to virtually zero
(C) capturing the voice of the customer
(D) preventing controllable defects

Answer: (d) Poke-yoke is about preventing controllable defects. This is a straight definition.

ASQ Six Sigma Green Belt Certification Poka-yoke Questions:

Which of the following is an example of mistakeproofing?

(A) Using an X Bar – R chart to prevent errors
(B) Using 100% inspection to detect and contain defects
(C) Using color coding as an error signal
(D) Having the team that created the errors repair them

Answer: (c) Using color coding as an error signal is an example of mistake proofing. (This isn’t the best example as anyone who suffers from color blindness can tell you! A better example is the pumps at the gas station. Notice how different fuel types get different shaped nozzles? That makes it very difficult to put the diesel pump into your gasoline tank!)

An X Bar R chart can reveal when your process is out of control, but will not make it mistakeproof. Using inspection to detect and contain defects can lead to better quality, but will not prevent those defects from occurring in the first place.

Having a team that created errors repair them may lead to learning and prevent those errors in the future – or it may not.

Having a color coded system where, for example, a only people with red badges are allowed on a certain floor of a hospital can prevent defects before they happen.

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