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Statistical Techniques and 6 Sigma Process Capability

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Author  Topic: Process Capability 
Tamashiro Lurker (<10 Posts) Posts: 4 
posted 09 July 1999 03:36 AM
In the Statistical process Control Reference Manual published by AIAG, capability falls under 2 categories; longterm and shortterm. On page 59, Longterm capability is denoted as Cpk and uses an estimated process standard deviation while shortterm capability is denoted as Ppk using the calculated standard deviation. It has been argued and published by many statistical program companies that Ppk is longterm and Cpk is shortterm. the reasoning for this confusion relies on the definition of what longterm and short term is and how it relates to the standard deviation. What is the definition of Cpk and Ppk and why do most software companies publish process capability contrary to the statements in the AIAG manuals? IP: Logged 
Don Winton Forum Contributor Posts: 498 
posted 09 July 1999 11:16 AM
There is some discussion of Cpk versus Ppk at: https://elsmar.com/ubb/Forum10/HTML/000028.html As far as the differences between AIAG and the software companies, I ain't gotta clue. Regards, IP: Logged 
Lassitude Forum Contributor Posts: 89 
posted 09 July 1999 12:15 PM
quote:Because the AIAG is not the only 'expert' or business in the world. Software companies are looking to sell to the world, not just Ford, GM and Chrysler. Specifically which software package are you talking about? IP: Logged 
Tamashiro Lurker (<10 Posts) Posts: 4 
posted 10 July 1999 12:44 PM
Specifically Minitab IP: Logged 
Marc Smith Cheech Wizard Posts: 4119 
posted 11 July 1999 01:41 AM
I have no experience with minitab to know. Can you cite the specific equation differences (or other details of the differences) you are asking about? IP: Logged 
Tamashiro Lurker (<10 Posts) Posts: 4 
posted 11 July 1999 03:52 PM
The equation for process capability is exactly the same as indicated in the AIAG reference manual. The clarification I'm looking for is in the definition of long term and short term. The AIAG reference manual does not define long term or short term, but seems to imply long term using Cpk. Cpk uses an estimated sigma, which implies it estimates capability based on trend and therefore may predicate future capability. Ppk, however uses a finite source of data for sigma, which implies capability for that particular set of data. In some of the software packages, I've noticed Cpk is referenced as shortterm and Ppk as longterm. What is the definition of longterm and shortterm capability and is longterm represented by Cpk or Ppk? IP: Logged 
Don Winton Forum Contributor Posts: 498 
posted 12 July 1999 01:09 AM
I am not familiar with Minitab either. I also do not have a copy of the AIAG manual. But, as I elaborated on in the forwarding thread I referenced above, I believe AIAG has produced more problems by creating this Ppk thing. I had never heard of it before the forwarding thread above, nor had I heard of shortterm or longterm capability studies. All process capability studies should use an unbiased estimate of standard deviation or calculated standard deviation as detailed in the CPK.PDF file located at the PDF Zone section of the Cove. It states: "To obtain an accurate estimate of the process spread (standard deviation), at least sixty data points are needed. If less that sixty data points are available, use the following formula with error correction factors given in Table 1." Basically, if there are less than sixty data points, you use an unbiased estimate of standard deviation corrected by the formula. If greater than sixty data are available, use the standard deviation calculated from the data. BTW, which symbol (s, sigmahat, shat, etc.) does the AIAG manual use in the formula for Ppk and Cpk to represent process spread? Regards,  Check Out dWizard's Lair: IP: Logged 
BRoyal Forum Contributor Posts: 22 
posted 17 July 1999 05:59 PM
Davis Bothe addresses the difference between Cpk and Ppk in his book "Measuring Process Capability." McGrawHill is the publisher. Be aware that this book will probably tell you more than you want to know about process capability Ben Royal IP: Logged 
Batman Forum Contributor Posts: 111 
posted 19 July 1999 03:20 PM
Hi Don! The AIAG manual uses [Greek]lower case Sigma with a hat for Rbar/d2 estimate from the XR chart. Lower case 's' for the sample standard deviation (n1) from a random sample. IP: Logged 
Marc Smith Cheech Wizard Posts: 4119 
posted 19 February 2001 06:34 AM
For a good thread on Cp, Cpk, Ppk, etc., see: https://elsmar.com/ubb/Forum10/HTML/000001.html IP: Logged 
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