Vol. 7, No. 03
from Professor Cleary
As for nirvana, Marty would do well to reconsider his absurd use of a term, with its implications for Buddhists of “perfect blessedness achieved by the extinction of individual existence.” On the other hand, it may be that Marty Graw may be about to achieve his own extinction, or at least that of his position as quality manager.
With respect to capability, what Marty failed to comprehend is that a process must be centered. That is, must be in the middle of the upper and lower spec, to achieve a Cp value of 1.00, or for a process to be producing virtually all good parts (or weights, in this case). See case A below:
In his situation, the process is not centered, as demonstrated in the chart below:
For Marty's example, the center of the process would be at 8, not 10. What results is a Cp of 1.17, but defects of 2.28 percent. If the process were centered, the Cp of 1.17 would indeed result in producing virtually all good parts. This is why the Cp can be a misleading indicator of the capability of the process.
Many would contend that the Cpk is a better index to use. Stay tuned next month, when we'll examine this possibility in the ongoing saga of dire dereliction in the dairy factory.
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