[原创]工程能力分析&正态性验证

Ppk is an index of process performance which tells how well a system is meeting specifications. Ppk calculations use actual sigma (sigma of the individuals), and shows how the system is actually running when compared to the specifications. This index also takes into account how well the process is centered within the specification limits.

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.