What's Difference For Cp, Cpk, Pp, and Ppk?

Process capability looks complicated. It does not have to be.

This one-page view breaks down the four most commonly confused metrics: Cp, Cpk, Pp, and Ppk.

Here is the simple way to remember them:
• Cp tells you the potential capability of a process, assuming it is centred and stable
• Cpk shows the actual capability, accounting for how far the mean has shifted
• Pp reflects overall performance using long-term variation
• Ppk shows actual long-term performance, including shifts and drift

A key takeaway many miss:
Cp and Cpk look at within-subgroup variation.
Pp and Ppk look at overall variation.

A process can look capable in the short term and fail in the long term.  That is why capability without stability is misleading.

Capability indices are not just numbers. They are signals that tell you how your process is really behaving.