Under the Potential capability heading we find the formulas for Cpk, and under Overall capability we find the formulas for Ppk. If we click Estimating standard deviation, we can find the formulas for the pooled standard deviation. The Methods and Formulas section shows the formulas Minitab uses. Then, from the Help window shown below, click the see also link, and then choose Methods and formulas: To see details of the formulas used, we can click the Help button in the lower-left corner. If we click the Estimate button in the dialog box that comes up, we can see the default methods used in Minitab:įrom the dialog box above, we can see that Minitab uses the Pooled standard deviation when subgroup sizes are greater than 1.
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Then, in my next post, I’ll show you how to calculate Cpk.įor data that follow the normal distribution, we use a Normal Capability Analysis ( Stat > Quality Tools > Capability Analysis > Normal). So in this post I will show you how to calculate Ppk using Minitab’s default settings when the subgroup size is greater than 1. Michelle Paret already wrote a great post about the differences between Cpk and Ppk, but it also helps to have a better understanding of the math behind these numbers. In technical support, we frequently receive calls from Minitab users who have questions about the differences between Cpk and Ppk.
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