Wednesday, August 21, 2013

Poka Yoke: Mistake Proofing to Reduce Errors

Some may say that it’s impossible to eliminate mistakes. And they are right; it is, most of the time. But, to be honest, they are missing the point.

Poka yoke, or mistake proofing, describes any behavior changing constraint that is built into a process to prevent an incorrect operation or act occurring.  The three aims of mistake proofing are:
  • To reduce the risk of mistakes or errors arising.
  • To minimize the effort required to perform activities.
  • To detect errors prior to them impacting on people, materials, or equipment.

Ideally, poka-yoke ensures that proper conditions exist before actually executing a process step, preventing defects from occurring in the first place. Where this is not possible, poka-yoke performs a detective function, eliminating defects in the process as early as possible.

This can be achieved using three rule. Following are three rules of poka yoke

  1. Make it impossible to get it wrong (Occurrence)
  2. Make it impossible to pass the defect onto the customer (Severity)
  3. Make it blatantly obvious that there is a defect (Detection)

In our normal lives we are all familiar with many different mistake-proofing concepts, such as windows that don’t open fully in order to prevent people falling out, self-closing fire doors, lights that turn themselves off when they detect no one is there, and so on. You will also have seen warning instructions and traffic lights, as well as having read work instructions, all of which aim to reduce the risk of error. However, you will also notice that some of these are more effective than others. For example, the windows that don’t open on the 15th floor of a hotel to prevent you falling out are likely to be more effective than leaving the windows able to open fully and providing an instruction pamphlet in one of the drawers that tells you to be safe when opening the windows. This shows there is a hierarchy of mistake proofing concepts, with a decreasing level of effectiveness.

The five levels of the hierarchy of mistake proofing are shown below.

  1. Eliminate – the most effective but also normally the most costly level involves eliminating the source of risk completely. In reality, it is very difficult to completely remove risk.
  2. Redesign – if you can’t eliminate the risk then you might want to try to replace it with a less risky process.
  3. Reduce – when it is not possible to redesign the problem you need to think about reduction techniques.
  4. Detect – here we are no longer trying to prevent mistakes: we are trying to detect  they have occurred.
  5. Mitigate – at this lowest level we are simply trying to reduce the damage caused by the mistake arising.

Some common examples of mistake proofing that most people should be aware of – and the levels they represent in the mistake proofing hierarchy:
  • Hard hats - mitigate
  • Drain hole at the top of a sink to prevent it overflowing - mitigate
  • Fuel low warning lights on cars – detect
  • Blood pressure monitoring equipment - detect
  • Safety glasses – reduce
  • Standard operating instructions – reduce
  • Filing cabinets that won’t allow you to open more than one drawer at a time – redesign
  • Garage door sensors that detect an obstruction – redesign
  • Irons that turn off automatically – redesign

The guiding principles of mistake proofing should be as follows:
  • People are fallible and even the best make mistakes.
  • Errors are inevitable.
  • Errors can be eliminated.
  • Error-likely situations are predictable, manageable, and preventable!
  • Events can be avoided by understanding the reasons mistakes occur and applying the lessons learned from past events.
  • Defects are preventable and zero defects can be achieved.

There is a perception in some organizations that particular people are “error proof” and that they do not and cannot make mistakes. If you believe that anyone is immune to making mistakes then you will be sorely disappointed in the very near future.

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  1. Is the hierarchy of mistake proofing you're presenting here something you came up with or referenced from something else? I've just noticed that it's very similar to the Hierarchy of Hazard Control ( and I was wondering if there's similar heritage

    1. Jason the hierarchy of mistake proofing is not my own. The exact source escapes me (course, book, webinar, etc). They are similar to those of the hierarchy of Hazard control. Not surprising I guess given the subjects. I imagine this thinking comes for Henry Ford and was later refined at Toyota by Shigeo Shingo.

      John Grout's website ( is always a resource I have found value regarding the subject.