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  APQP and PPAP
  PPAP Rev 3 Cpk/Ppk

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Author Topic:   PPAP Rev 3 Cpk/Ppk
SRTipton
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Posts: 3
From:Spencer, WV
Registered: Aug 1999

posted 14 February 2000 10:01 AM     Click Here to See the Profile for SRTipton   Click Here to Email SRTipton     Edit/Delete Message   Reply w/Quote
There appears to be some leeway for bulk suppliers like my company(ie, aluminum, ingot, coil) regarding PPAP Rev 3 Cpk/Ppk requirements. I'm not sure that my interpretation is correct but if a supplier-designated key characteristic is not in statistical control, then the reporting for PPAP should be by Ppk. We use a monthly Cpk/Ppk report to satisfy ongoing process capability (since our customer has no key characteristics identified for us). We have a prior interal audit nonconformance written against a process with a Cpk < 1.33. For the last 8 months the Cpk has been > 2 but this past month, the process was not in control (out of control points were investigeated with no assignable cause found)so, Ppk was used on the capability report. Now, the Ppk is < 1.33. Can I close the nonconformance based on the prior 8 months or should I leave it open and shoot for > 1.33 on both Cpk and Ppk, even though this is a supplier designated key characterstics.

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Marc Smith
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From:West Chester, OH, USA
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posted 15 February 2000 06:55 AM     Click Here to See the Profile for Marc Smith   Click Here to Email Marc Smith     Edit/Delete Message   Reply w/Quote
quote:
Now, the Ppk is < 1.33. Can I close the nonconformance based on the prior 8 months or should I leave it open and shoot for > 1.33 on both Cpk and Ppk
I would think that if the process has been in control for 8 months and only briefly went out and is now back in and your investigation turned up nothing (we know we missed something, no?) I would think you could close it out. Problem is, we know there is an assignable cause - it's not noise - what will you do if it happens again in 8 months? Will anything 'trigger' you to come back to this incident (recurrence)?

Do see https://elsmar.com/ubb/Forum10/HTML/000001.html

From www.pqsystems.com/cpk-ppk.htm :

Cpk or Ppk: Which should you use?

Your customer has asked you to report the Cpk of the product you are sending. You know that to compute the Cpk, you need to have the product specifications, and that you need to have the mean and sigma. As you gather the information, someone asks, "which sigma do they want?"

You know that Cpk is calculated by dividing by 3 sigma. But which sigma should you use, estimated or calculated? Which is correct? Which would you report? Naturally, most of us would use the sigma that makes the Cpk look the best. But the sigma that makes the Cpk look best may not accurately reflect what you or your customer need to know about the process.

Confusion over calculating Cpk by two different methods is one reason that a new index, Ppk, was developed. Ppk uses the calculated sigma from the individual data.

Given that Ppk uses the calculated sigma, it is no longer necessary to use the calculated sigma in Cpk. The only acceptable formula for Cpk uses the estimated sigma.

In 1991, the ASQC/AIAG Task Force published the "Fundamental Statistical Process Control" reference manual, which shows the calculations for Cpk as well as Ppk. These should be used to eliminate confusion about calculating Cpk. So which value is best to report, Cpk or Ppk? Although they show similar information, they have slightly different uses.

Estimated sigma and the related capability indices (Cp, Cpk, and Cr) are used to measure the potential capability of a system to meet customer needs. Use it when you want to analyze a system's aptitude to perform.

Actual or calculated sigma (sigma of the individuals) and the related indices (pp, Ppk, and Pr) are used to measure the performance of a system to meet customer needs. Use it when you want to measure a system's actual process performance.

[This message has been edited by Marc Smith (edited 23 February 2001).]

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SRTipton
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From:Spencer, WV
Registered: Aug 1999

posted 15 February 2000 07:45 AM     Click Here to See the Profile for SRTipton   Click Here to Email SRTipton     Edit/Delete Message   Reply w/Quote
quote:
Problem is, we know there is an assignable cause - it's not noise - what will you do if it happens again in 8 months? Will anything 'trigger' you to come back to this incident (recurrence)?
The Trigger would again be the monthly ongoing capability report. If the out of control condition continues in the next report, then we know we have lost our in-control process.

[This message has been edited by Marc Smith (edited 15 February 2000).]

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