posted 06 September 1999 09:35 AM            

Also by the definition of Ppk and Cpk formula, Ppk value cannot be greater than Cpk since the "sigma" in Ppk includes both common and special causes of variation. And yet the acceptance for Ppk is supposedly 1.67 but for Cpk it's 1.33.

posted 01 October 1999 10:42 PM            

David McGan
Contributor posted 03 August 1997 11:08 AMÊÊ
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I'm almost embarrassed to ask the question, but I've not been able to find a good, clear explanation of the real difference between the Ppk and Cpk indices. Ppk, I know, is usually specified for short-term study results, and uses the calculated standard deviation in its determination. Cpk, on the other hand, is used for long-term study results, and uses the estimated standard deviation in its calculation. But can anyone give me the statistical rationale for this?
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I'm a generalist. Take a look at https://elsmar.com/ubb/Forum10/HTML/000021.html and see if that helps. Also see https://elsmar.com/ubb/Forum10/HTML/000024.html

If those don't help, come back to this thread, or post a reply in either (or both) of the threads.

Any comments, Don?

posted 03 October 1999 11:49 AM               

Any comments, Don?

Hmmm. Lets see. First, thanks for Îlighting the bulb.â I have been busy this past few weeks and had forgotten this was posted. My apologies for the delayed response.

First, as seen in the links Marc gave I have made my feelings about this whole Ppk thing known. I feel it is just a gimmick that the AIAG group uses to muddy the water. Just MHO.

Typically, Ppk uses sample standard deviation (s) while Cpk uses an unbiased estimate if sigma (sigma hat). The reason (I suppose) is that there may be occasions where this is more applicable. This is not true. Even with small sample sizes, an unbiased estimate of sigma can still be obtained.

As far as the 2nd edition of the PPAP manual goes, I ainât gotta clue. As I have stated before, I do not do the QS thing.

Regards,
Don

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