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  Statistical Techniques and 6 Sigma
  CP & CPK'S

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Author Topic:   CP & CPK'S
MSHELLEY
Lurker (<10 Posts)

Posts: 4
From:Wilson, NC 27893
Registered: Aug 98

posted 28 January 2000 01:35 PM     Click Here to See the Profile for MSHELLEY   Click Here to Email MSHELLEY     Edit/Delete Message   Reply w/Quote
WE REGULARLY COLLECT DATA FROM CONTROL CHARTS AND COMPUTE THE CAPABILITY INDEXES Cp & Cpk. WE ALSO COLLECT DATA FROM DOCK AUDITS AND COMPUTE THE SAME INDEXES. WHY DO THE ONES FROM THE CHARTS ALWAYS HIGHER. ASSUMPTION IS THAT ALL DATA IS GOOD.

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Laura M
Forum Contributor

Posts: 299
From:Rochester, NY US
Registered: Aug 1999

posted 28 January 2000 02:09 PM     Click Here to See the Profile for Laura M   Click Here to Email Laura M     Edit/Delete Message   Reply w/Quote
If I'm understanding the scenario correctly, I would wonder how are you calculating the std. dev? If you are using r-bar /d2 for the control charts, and experience mean shifts, then the est. of you std dev. can be lower than the dock audit if all the parts are thrown into one "subgroup" and std dev. calculated for the lot. Lower std dev, higher Cpk. Do the process charts show control?

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Don Winton
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From:Tullahoma, TN
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posted 30 January 2000 08:21 PM     Click Here to See the Profile for Don Winton   Click Here to Email Don Winton     Edit/Delete Message   Reply w/Quote
Laura brings up a good point. I also would question the standard deviation calculations on the dock audits. Typically, when your sample sizes are less than 60, you must use an unbiased estimate of sigma. With control charts, this is not a problem, but with audits, caution must be exercised.

For more information regarding process capability, go to https://elsmar.com/pdf_files and download CPK.PDF. This paper explains the unbiased estimate of sigma. You can also visit my web site below for other stuff.

Regards,

Don

[This message has been edited by Marc Smith (edited 19 February 2001).]

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Marc Smith
Cheech Wizard

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From:West Chester, OH, USA
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posted 19 February 2001 06:35 AM     Click Here to See the Profile for Marc Smith   Click Here to Email Marc Smith     Edit/Delete Message   Reply w/Quote
For a good thread on Cp, Cpk, Ppk, etc., see: https://elsmar.com/ubb/Forum10/HTML/000001.html

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