So, what is meant by the words process capability? For a process. Customers want to know your process capability. This gave rise to numerical You hear the term process capability a lot these days. Since you are comparing two ranges - the range of output from the process to the range of the specs - it is natural to ask questions about how much these two ranges are different or overlap. Process capability is the range of values that you can expect to get from the process over an extended time period.
#Excel sheet cpk series#
Know what it is? Our first two blogs in this process capability series answered two questions: What is process capability? This blog answers the next question: What is Ppk? There were two parts to the answer. Why are those different? The Cpk is above 1. Looks like your supplier is really performing for you.
#Excel sheet cpk software#
Once you determine which capability index you will use, it can easily be calculated using software such as SQCpack.Your supplier has sent you the process capability chart you requested. Use it when you want to measure a system's actual process performance. 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 analyze a system's aptitude to perform. 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. So which value is best to report, Cpk or Ppk? Although they show similar information, they have slightly different uses. These should be used to eliminate confusion about calculating Cpk. The only acceptable formula for Cpk uses the estimated sigma. Given that Ppk uses the calculated sigma, it is no longer necessary to use the calculated sigma in Cpk. Ppk uses the calculated sigma from the individual data. But the sigma that makes the Cpk look best may not accurately reflect what you or your customer need to know about the process.Ĭonfusion over calculating Cpk by two different methods is one reason that a new index, Ppk, was developed. Naturally, most of us would use the sigma that makes the Cpk look the best. Which is correct? Which would you report? Cpk & Ppk and Differences between Minitab and an Excel Spreadsheet Results But which sigma should you use, estimated or calculated? You know that to compute the Cpk, you need to have the product specifications, and that you need to have the mean and sigma.Īs you gather the information, someone asks, "Which sigma do they want? You know that Cpk is calculated by dividing by 3 sigma. Your customer has asked you to report the Cpk of the product you are sending. A free online reference for statistical process control, process capability analysis, measurement systems analysis, and control chart interpretation, and other quality metrics.