Make No Mistake

by Edwin B. Smith III

Mistake proofing tools provide low cost, effective defect prevention and operator feedback. They can stop mistakes from being made or make mistakes obvious at a glance. Such tools either prevent the special causes that result in defects or inexpensively inspect each item produced to determine whether it’s acceptable or defective.

Ideally, the mistake proofing device, not the operator performing the assembly step, will inspect the product so any mistakes become instantly apparent. The operation cannot be completed unless the defect free condition is satisfied. For example, a floppy disk cannot be inserted upside-down, but the user will not become aware of the mistake proofing feature unless he or she attempts to insert the disk upside-down.

Mistake proofing should be the cornerstone of any manufacturing based quality system. When designing a manufacturing system, you will encounter user and operator errors, but you must remember they are rarely single, independent events; they are usually part of a larger sequence.

Complex, technology based solutions that inspect quality into the process are often the first considered, but they aren’t always the best. Instead, you should work to prevent errors and design the production system to handle them when they do occur by employing techniques such as forgiveness of user actions or forced next step. This analysis will lead you to redesign the process to include mistake proofing tools over traditional inspection and statistical process control tools.


Mistake proofing is best implemented in a team environment and should involve shop floor operators and their supervisors, the engineering department and the voice of the customer. Unfortunately, this approach is not often used. Instead, engineers and managers force-feed mistake proofing devices to the production floor. These devices meet with resistance, and the effort ends up providing no process improvement. Only when everyone from the shop floor to senior management receives training on the mistake proofing concept and becomes directly involved in its implementation will the program be successful.

The Cumbersome Clip

Many components cannot tolerate the high temperatures of the soldering process and are either destroyed or degraded when no thermal controls are used. In one example, a filter had to be soldered to the end of a cable. Heat from the soldering iron was damaging the filter, so the production engineer offered heatsink clips to the operators.

The heatsink clip was attached to the filter leads to dissipate the heat before it reached the sensitive filter body. This method worked if the clip was placed correctly on the filter lead, but the filter’s round shape made it cumbersome and awkward. This, combined with the need to carefully place the clips, discouraged the workers from using them. The operators deviated from the procedure and attempted to stabilize the filters in their own peculiar ways.

A cross functional team of people from program management, quality, engineering and production met to review and solve this problem. The team developed a mistake proofing concept and produced a two-function fixture, which held the cable steady during soldering and provided an automatic heatsink effect. The new fixture acted as a clamp for soldering stability, and its built-in heatsink capability dissipated heat away from the filter.

When the fixture was used, it was impossible to build the product without simultaneously heatsinking the filter leads. The operators readily accepted the fixture because it helped them without requiring any extra effort. Customer failures dropped to zero after its implementation.

The team approach allowed production to have a voice in implementing a solution to the excessive heat failure mode. And because program management was on the team, the customer was kept advised as the solution moved toward implementation.

Mistake proofing can reduce the cost of poor quality and provide a more rapid introduction of needed process improvement solutions. Most mistake proofing devices are simple and inexpensive. Adding a team approach to mistake proofing implementation ensures the voice of the shop floor and the voice of the customer can be heard throughout the implementation process.


  1. Sawyer, Dick, Do It By Design: An Introduction to Human Factors in Medical Devices, FDA, 1996.
  2. Shingo, Shigeo, Zero Quality Control: Source Inspection and the Poka-Yoke System, Productivity Press, 1986.
  3. Wiegman, Douglas, A Human Error Analysis of Commercial Aviation Accidents Using the Human Factors Analysis and Classification System (HFACS); DOT/FAA/AM-01/3, 2001.

EDWIN B. SMITH III is the operations manager at Hart InterCivic in Lafayette, CO. He earned an MBA from the University of Phoenix in Denver. Smith is a member of ASQ and a certified quality engineer, quality auditor and quality manager.

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