Cp/Cpk are process capability indices used to compare a process’s measured variation with its specification limits. They are commonly used in manufacturing quality, statistical process control, and capability studies to assess whether a stable process can produce output within defined tolerances.
Cp estimates potential capability by comparing the width of the specification range with the process spread. It does not account for whether the process average is centered between the limits. Cpk accounts for both spread and centering by evaluating how close the process mean is to the nearest specification limit.
In industrial systems, Cp and Cpk may be calculated from inspection or measurement data collected through quality systems, MES, SPC software, or controlled data analysis workflows. The data should be traceable to the relevant characteristic, measurement method, specification revision, and production process. First article inspection data may support context, but it is not a capability study by itself unless the data set and process conditions are statistically appropriate.
Cp/Cpk should not be confused with Pp/Ppk. Cp/Cpk are typically associated with within-process capability for a stable process, while Pp/Ppk are commonly used to describe overall process performance over a broader data set. Actual interpretation depends on the statistical method, sampling plan, process stability, and measurement system quality.