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What Is a Stable Process?
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Posted by: Kim Niles
Posted on: Monday, 11th June 2001, 12:00 AM.
I think it's funny that we keep on debating around the bush on issues that have an underlying theme of undefined process stability. I recently posted under "Any one could tell me when I should use CPK or PPK? THKS!" the following:
+~+~+
Montgomery states in his book: Montgomery, Douglas. C. "Introduction to Statistical Quality Control". Wiley & Sons, Inc. New York. 2001. 4th ed. Pg 372, that in 1991 the Automotive Industry Action Group (AIAG) was formed with one of their objectives being to standardize industry reporting requirements. He says that they recommend Cpk when the process is in control and Ppk when it isn't. Montgomery goes on to get really personal and emotional about this which is unique to this page of this book and other books I have of his. He thinks Ppk is baloney as he states "Ppk is actually more than a step backwards. They are a waste of engineering and management effort - they tell you nothing".
While Montgomery gets frustrated over the use of Ppk, he does a poor job of explaining what a stable process is. I respect his works just the same as he alone has even attempted to try to explain the difference between a stable process and a non-stable one.
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This argument as well as numerous others that you can find on this site regarding use of Cpk / Ppk metrics, the validity of Six Sigma shifts, process capability, SPC, etc. all reflect our lack of definition for what is process control.
So, how can we define process stability and or process control? Perhaps we can agree on some given amount of process shifting (1.5 sigma)? Perhaps we can agree that a stable process is that where it's Cpk values are above 1.67? Perhaps some combination of these or other events needs to take place such as three consecutive Cpk samples over 1.67, etc.
Until we can define what a stable process is, we are doomed to argue forever all use of any statistical metric.
For the love of a all science, please help!!
Sincerely,
KN - http://www.znet.com/~sdsampe/kimn.htm
Message Thread:  | | Paranoia by Kim Niles on Friday, 22nd June 2001 |
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