Quantifying how well a process meets specifications.
Process capability measures whether a process can consistently produce output within specification limits. The process capability index (Cp, Cpk) and process performance indices (Pp, Ppk) answer: “Can this process meet the customer’s requirements?” I use these metrics to qualify new processes, validate improvements, and support statistical process control (SPC) and quality systems.
Cp reflects potential capability (spread of the process relative to the tolerance band); Cpk adds centering, so it reflects actual capability. Pp and Ppk are the performance equivalents when the process is not necessarily in statistical control. A Cpk ≥ 1.33 is commonly required for critical characteristics in manufacturing.