Process performance index, built like Cpk but using the overall standard deviation of all the data, so shifts between subgroups, lots and days count against it. Its partner Pp is the matching version of Cp. Ppk reflects what the customer actually received, while Cpk shows what the process could deliver if those between-subgroup shifts were removed. Automotive initial process studies commonly look for a Ppk of 1.67 before launch.
Cpk looks great at 1.8, but Ppk is 1.1. Lot-to-lot variation is eating us, go look at the raw material.
Also heard as
- process performance index
- performance index
Related terms
Cpk
Process capability index that accounts for centering: distance from the mean to the nearest spec limit in units of 3 sigma.
Cp
Process capability ratio: the specification width divided by six process standard deviations, ignoring centering.
Process capability
How well a stable process's natural spread fits inside the specification limits, usually expressed as an index.
Variance components
Breakdown of total variation into the shares contributed by each source, such as lot, wafer, site and measurement.