What is the difference between Ppk and Cpk


Brad Lee

(I crossed post this from the Stat Techniques Forum as I think the topic applies to both.)

The 2nd edition of PPAP says we are suppose to calculate "Preliminary Process Capability" in the form of Ppk, and yet the SPC Ref. Manual says Ppk is "Process Performance" while Cpk is "Process Capability". So what gives?

Also by the definition of Ppk and Cpk formula, Ppk value cannot be greater than Cpk since the "sigma" in Ppk includes both common and special causes of variation. And yet the acceptance for Ppk is supposedly 1.67 but for Cpk it's 1.33.

Anyone's got an insight to this?????


The way I interpret this is Ppk is a one time, early-in-the-project short capability of a few hours or so. The Cpk is usually calculated from a longer timeframe, days or weeks for example. If you have a "short run" of a couple of hours with a capability index of less than 1.67, it is presumed that a longer study will result in added variation, thus lowering the eventual long term capability index. I believe this is why the higher requirement for the Ppk.
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