What is the Difference Between PPK & CPK?
Ger de Waard, Management Consultant, Netherlands
In the Six Sigma quality methodology, process performance is reported to the organization as a sigma level. The higher the sigma level, the better the process is performing.
Another way to report process capability and process performance is through the statistical measurements of Cp, Cpk, Pp, and Ppk.
• Cp = Process Capability. A simple and straightforward indicator of process capability.
• Cpk = Process Capability Index. Adjustment of Cp for the effect of non-centered distribution.
• Pp = Process Performance. A simple and straightforward indicator of process performance.
• Ppk = Process Performance Index. Adjustment of Pp for the effect of non-centered distribution.
Cp and Cpk are for computing the index with respect to the subgrouping of your data (different shifts, machines, operators, etc.), while Pp and Ppk are for the whole process (no subgrouping).