Stats tip: What everyone ought to know about Cpk vs. Ppk

Matt SavageIn a class on capability analysis that I recently taught, several participants asked: “What is the difference between Cpk & Ppk?”

The quick answer is… ‘1 letter’. The mathematical answer is that each statistic uses a different calculation for the standard deviation.

The practical answer is…’it depends’. Now I know, some of you are saying statistics (math) isn’t subjective. I mean, we’re dealing with numbers here! Yes, the numbers are real and yes, you should trust the results. (Unless, of course, someone like Bernard Madoff is gathering the data for you.)

So getting back to the practical answer, if the process is in a state of statistical control, then it really doesn’t matter which statistic you use (Cpk or Ppk) because the calculation results will be so similar to each other. Provided you have created a control chart with “good data” (no Bernie collecting the data), and the control chart shows the process in a state of statistical control, you will find that Cpk is in the same ballpark as Ppk. And since the purpose of both indices is to give you a feel for how good (or bad) the process is behaving, either will do. Therefore, no significant difference exists between Cpk & Ppk. Now, if the process is not in control, all bets are off. When the process is not in control, do not put your trust in Cpk. But I’ll leave that discussion for another day.

One thought on “Stats tip: What everyone ought to know about Cpk vs. Ppk

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