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  Statistical Techniques and 6 Sigma
  Ppk is Preliminary Process Capability ?

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Author Topic:   Ppk is Preliminary Process Capability ?
Brad Lee
Lurker (<10 Posts)

Posts: 3
Registered: Sep 1999

posted 06 September 1999 09:35 AM     Click Here to See the Profile for Brad Lee     Edit/Delete Message   Reply w/Quote
The 2nd edition of PPAP says we are suppose to calculate "Preliminary Process Capability" in the form of Ppk, and yet the SPC Ref. Manual says Ppk is "Process Performance" while Cpk is "Process Capability" ·.so what gives?

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.

Anyone's got an insight to this???

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Cheech Wizard

Posts: 46
Registered: Feb 99

posted 01 October 1999 10:42 PM     Click Here to See the Profile for admin     Edit/Delete Message   Reply w/Quote
First, although some time ago, I found this in a search from 1997:

David McGan
Contributor posted 03 August 1997 11:08 AMÊÊ
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?

I'm a generalist. Take a look at and see if that helps. Also see

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

Any comments, Don?

[This message has been edited by admin (edited 01 October 1999).]

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Don Winton
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Posts: 498
From:Tullahoma, TN

posted 03 October 1999 11:49 AM     Click Here to See the Profile for Don Winton   Click Here to Email Don Winton     Edit/Delete Message   Reply w/Quote

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.


Just the ramblings of an Old Wizard Warrior.

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