Although many techniques have been developed to prevent or control mistakes, most of these techniques are relatively ineffective. Effective mistake-proofing can not be developed without a sound understanding of the true characteristics of mistakes. A mistake occurs when a required action is not performed or is performed incorrectly, a prohibited action is executed, or information essential for an action is not available or is misinterpreted.

Mistake Characteristic: Mistakes are extremely rare random events.

Although a few mistakes occur more frequently, each specific type of error is undetected about once in 10,000 to once in 100,000 operations in modern production environments

Mistake-Proofing Attribute: Mistake-proofing requires 100% inspection

• It is impossible to detect and control rare random events with sampling inspection. Since, 100% traditional inspection is too expensive and not 100% effective in detecting nonconforming product, mistake-proofing methods based on poka-yoke are essential and the only practical solution.

Mistake-Proofing Attribute: Mistake-proofing must be inexpensive

• Because mistakes are rare events and many different types of mistakes must be controlled, companies can not afford to spend large sums of money on each mistake-proofing device. Toyota has a goal of implementing each mistake-proofing device for under $150.

Mistake Characteristic: Many different mistakes can occur

So many types of mistakes can occur that, collectively, mistakes are the dominant source of defects in modern products and services even though each specific type of mistake is a rare event. For example, in the manual fabrication and assembly of the illustrated cable clamp, the following 17 errors (10 types) could occur:

Bolt is missing from assembly (2) Bolt torque is too high or two low Holes for bolts are not drilled (4) Cable clamp is assembled backwards Drilled hole for bolts are not tapped (2) Large thread on cable clamp is not formed

Nut is not tapped Bolt is not threaded (2) Bolt is not tightened (2) Cable clamp is omitted

Mistake-Proofing Attribute: Many mistake-proofing devices are needed

• Toyota has an average of 12 mistake-proofing devices at each workstation. To illustrate how many devices are required to mistake-proof a simple process,
  see fueling examples.

Mistake-Proofing Attribute: Mistake-proofing must be inexpensive

• Because mistakes are rare events and many different types of mistakes must be controlled, companies can not afford to spend large sums of money on each mistake-proofing device. Toyota has a goal of implementing each mistake-proofing device for under $150.

Mistake Characteristic: Mistake-proofing methods are not equally effective

Controlling causal factors such as noise, lighting, and clutter will reduce mistake rates. However, the most significant causal factors are not directly or completely controllable by employers, including: forgetfulness, distraction, fatigue, insomnia, medication side effects, drug abuse, or stress resulting from a divorce, death in the family, a misbehaving child, or co-worker conflicts.

Mistake-Proofing Attribute: Outcome intervention is best

• The best mistake proofing physically prevents errors or detects when a mistake is about to occur or has occurred. Thus, these techniques intervene to block undesired outcomes rather than controlling casual factors.

Mistake-Proofing Attribute: Prevention is better than detection

• Preventing mistakes is better than detecting mistakes, which is better than detecting defects. If a mistake is not detected until a defect is generated, rework will be required or the hardware must be scrapped. Thus, where possible, it is always better to detect or control the mistake before a defect is generated. Similarly, there will be less wasted effort if mistakes are prevented rather than detected.

Mistake-Proofing Attribute: Control, Shutdown, or Warn

• Because resources may be wasted if a process is shutdown, it is better to control mistakes. If a process is shutdown, however, the problem must be addressed to proceed. Hence, shutdown provides a more positive control of mistakes than warnings, which can be ignored.

Mistake Characteristic: Virtually all customer complaints result from mistakes

A study conducted by an aerospace company determined that mistakes were the source of 50 to 70 percent of the nonconformities leaving the production plant. However, every customer requested field service action other than scheduled maintenance was traced to mistakes with only one exception being attributed to excessive variation. This multi-year study addressed actions required to support more than 10,000 complex units in the field.

Mistake-Proofing Attribute: The Most Important Quality Initiative

• Only mistake-proofing effectively controls mistakes. Significant quality initiatives other than mistake-proofing have marginal impact on the customer perception of quality since mistakes are the dominate source of customer problems.