What does Cpk measure in process capability?

Cpk measures how well a process can produce output within specified limits, taking into account how centered the process is relative to those limits. The key aspect of Cpk is that it accounts for natural variability in the process by evaluating both the mean of the process and its spread, specifically assessing its capability at the worst-case scenario.

When the process is centered, Cpk will be equal to Cp, which measures the potential performance of the process based on the specification limits without considering the mean position. Therefore, Cpk provides a more realistic measure of process capability because it reflects not only the maximum potential performance but also the actual operational conditions where the process may be offset from the ideal target.

This means that when analyzing a process's performance, understanding Cpk is crucial as it can indicate whether corrective action is needed to center the process or improve variability to stay within specification.