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ST: Calculating capability indices with one specification
Viewed 6916 times since Fri, Nov 9, 2007
Revision Date: 2005-09-06 by Matt Savage  The following formula for Cpk is easily found in most statistics books. Cpk = Zmin / 3 Zmin = smaller of Zupper or Zlower Zupper = [(USL – Mean) / Estimated sigma*] Zlower = [(Mean – LSL)... Read More
ST: Is Cpk the best capability index
Viewed 2523 times since Fri, Nov 9, 2007
Revision Date: 2005-09-06                                     ... Read More
ST: The capability index dilemma - Cpk, Ppk, or Cpm
Viewed 2404 times since Fri, Nov 9, 2007
Revision Date: 2005-09-06 by Matt Savage Lori, one of our customers, phoned the other day to ask if Cpk is the best statistic to use in a process that slits metal to exacting widths. I too wondered what index would be best suited for her... Read More
ST: Cpk or Ppk - Which should you use
Viewed 2336 times since Fri, Nov 9, 2007
Revision Date: 2005-09-06 by Matt Savage 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... Read More
ST: How can Cpk be good with data outside the specification
Viewed 2256 times since Fri, Nov 9, 2007
Revision Date: 2005-09-06 by Matt Savage A customer who called our technical support line recently could not understand why his Cpk was above 1.0 when his data was not centered between the specifications and some of the data was outside the... Read More
ST: Should you calculate Cpk when your process is not in control
Viewed 2162 times since Fri, Nov 9, 2007
Revision Date: 2005-09-06 by Matt Savage The AIAG Statistical Process Control reference manual (p. 13) states: "The process must first be brought into statistical control by detecting and acting upon special causes of variation. Then its... Read More
ST: How do we determine process capability if the process isn’t normal
Viewed 1996 times since Fri, Nov 9, 2007
Revision Date: 2005-09-06 Cp, Cpk, Pp, and Ppk do not necessarily rely on the normal distribution. If any of these indices increases, you know that the process capability has improved. What you do not know is how that improvement translates... Read More