Ppk

In practice

Ppk compares the overall spread of all the data, including shifts between subgroups such as a new material lot or a different shift, with the specification, and takes into account how well centred the process is. Pp asks the same question as if the process were perfectly centred. Cpk uses only the short-term spread within subgroups, so it shows potential, while Ppk shows what the customer actually received. If Cpk is much higher than Ppk, the process is moving between subgroups. Ppk is often required in initial studies of a new process, such as automotive PPAP.