Question 1

When should I use Ppk instead of Cpk?

Accepted Answer

Use Ppk for initial process qualification (PPAP), long-term performance reporting, and when data is collected over extended periods covering multiple setups, operators, and material lots. Use Cpk for ongoing SPC monitoring with rational subgroups. Many automotive OEMs require both: Ppk ≥ 1.67 for initial approval, Cpk ≥ 1.33 for ongoing production.

Question 2

Why is my Ppk much lower than my Cpk?

Accepted Answer

A large gap between Cpk and Ppk indicates significant special cause variation between subgroups — such as shift-to-shift differences, tool wear, material lot changes, or temperature drift. The process may look stable within each subgroup (good Cpk) but drifts significantly between subgroups (poor Ppk). Investigate and eliminate the between-subgroup variation sources.