Cp Cpk Formulas and the Mistakes in Homemade Templates
Creating homemade Cp and Cpk templates often results in incorrect values. There are many, many mistakes you might make without realizing. Here are a few examples.
A customer sent me their home grown template for calculating Cp and Cpk and wondered why the QI Macros got such radically different values. It was easy to see from their data that they were using standard deviation, not Sigma estimator (Rbar/d2) to calculate Cp and Cpk. Use Stdev to calculate Pp and Ppk, not Cp and Cpk:
They had run the QI Macros histogram on two columns of data, one measured at 0 degrees and one measured at 90 degrees.
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