Poka-Yoke or Error Proofing or Mistake Proofing – is a technique for eliminating errors by making it impossible to make errors in the process. It is often considered the best approach to process control. It is a Process analysis tool.

It works on the principle – “It is good to do it right the first time. It is even better to make it impossible to do it wrong the first time”.

Error Proofing can be preventive (prevent the error from occurring) or detective (detect the error if it happens) in nature and is usually implemented in one of the following ways.

1. Warning: audio and/or visual warning either before the error occurs or after it has occurred.
2. Control: errors are not possible or even if an error occurs, it is not allowed to move to the next step.
3. Shutdown: machine or process shutdown when an error is about to happen or as soon as an error occurs.

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 methods, setting functions, and regulatory functions expanded on below.
  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 and Regulatory Functions

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 it weighs an item to ensure completeness.
  • A fourth setting function is sometimes added, information enhancement, which 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).

Baka-Yoke

Baka yoke is the Japanese term for “fool proofing” or “idiot proofing”. Needless to say, it is not the most politically correct of terms, and has been replaced in common use by Poka-yoke, or “mistake proofing”

Baka-Yoke is (idiot or fool proofing) a manufacturing technique of preventing mistakes by designing the manufacturing process, equipment, and tools so that an operation literally cannot be performed incorrectly; an attempt to perform incorrectly, as well as being prevented, is usually met with a warning signal of some sort.