Cpk: Process Capability Index. Adjustment of Process Capability (Cp) for the effect of non-centered distribution. Cpk tells you what the process is CAPABLE of doing in the future, assuming it remains in a state of statistical control.
Ppk: Process Performance Index. Adjustment of Process Performance (Pp) for the effect of non-centered distribution. Ppk tells you how the process has performed in the past. You cannot use it to predict the future because the process is not in a state of control.
Differences Between Cpk and Ppk
1. Cpk is the potential of a process to meet a specification (short term). It is used to measure the potential capability of a system to meet customer needs.
2. Ppk is how the process actually did (long term). It is used to measure the performance of a system to meet customer needs. Use it when you want to measure a system’s actual process performance.
3. Cpk considers variation within subgroups (common cause), Ppk considers total variation, which means variation between and within subgroups ( common cause and special cause).