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The Quality Technology Corner by John J. Flaig, Ph.D. "The Process is Out of Control"
When the process is producing defective product, the panicked exclamation is often heard from those in charge, "The Process is Out-of-Control". |
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Their knee jerk explanation exposes an interesting distinction between the meaning and usage of words in the language of common discourse and the technical language of the engineer. The engineer has a completely different understanding of the situation in that the production of defective product may or may not imply that the process is unstable. In fact, the process may be perfectly in-control and yet be producing 100% defective product.
The basis for this confusion lies in the fact that when output of a system is bad, we describe the system as "out-of-control" meaning that it is not producing what we want. However, the engineer looks at the behavior of the system in terms of the average and variation in the characteristics that determine product acceptance. If the mean and variance are stable, the engineer concludes that the process is in-control, and this is true even if 100% of the product fails to meet the specifications.
So why is it important to understand this distinction? Because, if you think the process is "out-of-control", then you think the system is in a chaotic state. There are no inferences for corrective action that can be made if the system is in chaos. However, if you observe that the process is stable but producing defective product because the mean is too low, then you may be able to adjust the mean and fix the problem. Likewise, if the variation is stable but too large resulting in defective product, then you may be able to redesign the process to reduce the variation and improve the yield.
John J. Flaig, Ph.D.
Fellow of the American Society for Quality
Managing Director
Applied Technology
Tel: 408-266-5174
E-mail: JohnFlaig@yahoo.com
Web: www.e-AT-USA.com
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