Why a process can post an excellent Cp and still be quietly manufacturing a pile of defective parts.
Cp and Cpk both get reported as "process capability," get plotted on the same dashboard, and get quoted in the same breath — so it's easy to assume they're measuring roughly the same thing. They aren't. Cp only ever asks whether a process's natural variation is narrow enough to fit inside the tolerance window. Cpk asks the question that actually predicts defects: narrow enough, andsitting where it's supposed to. A process can ace the first question and badly fail the second.