Ppk measures how a process actually performed against its tolerance over a period of time. The formula has the same shape as Cpk: the distance from the mean to the nearest specification limit, divided by three standard deviations. The difference is that Ppk uses the overall standard deviation of all the data, so shift-to-shift drift, tool wear and material changes are all included. Ppk is usually equal to or lower than Cpk, and a big gap between them points to variation between subgroups.
Example: Using the same shaft (9.94 to 10.06 mm, mean 10.01 mm), the overall standard deviation across a week of data is 0.015 mm. Ppk = (10.06 − 10.01) ÷ (3 × 0.015) = 1.11, while Cpk from within-subgroup data was 1.67. The gap suggests the process shifts between setups.
Ppk is what the customer actually received, which is why it often appears in PPAP submissions and launch studies. When Ppk is much lower than Cpk, the fix is usually in setups, tool changes or material lots rather than in the machine itself.
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