Definition
A process capability index that reflects both spread and centering at once: min[(USL−μ)/3σ, (μ−LSL)/3σ]. Cp looks at spread only; the gap between Cp and Cpk is the degree of off-centering, which tells you whether to re-center the process or reduce its variation. (Note: within DBS, "CP" separately means "Certified Practitioner" — spell out Cp/Cpk in full when discussing process capability to avoid confusion.)
In detail
Process Capability Index (Cpk) measures process performance within specification limits, considering both data spread and centering offset. By comparing the specification limits with the process mean and standard deviation, it reflects whether the process can meet tolerances while remaining stable. The difference between Cpk and Cp is that Cp only considers spread, while Cpk also accounts for off-centering. The gap between them reveals the adjustment direction: if Cpk is much smaller than Cp, the priority should be to adjust the process center rather than reduce variation. Cpk typically outputs a numerical value used to assess whether the process meets customer requirements.
Example in practice
In a machining workshop, the Cpk value for a critical dimension is only 0.8, below the customer requirement of 1.33. The engineer examines the data and finds that Cp is 1.5, indicating acceptable spread, but the mean is off target. They decide to first adjust the tool compensation to bring the center back to target, then recalculate Cpk. If still insufficient, they consider improving equipment precision. The final output is a process capability report guiding process adjustments.