In the manufacturing industry, one way to report and evaluate the process capability and process performance is through the statistical measurements, like Cpk.
Definition: Cpk = Cpk = Process Capability Index. Adjustment of Cp for the effect of non-centered distribution. Cpk measures how close you are to your target and how consistent you are to around your average performance.
Cpk measures two things: 1) how close the mean of the readings are to the center of the lower and upper spec limits (ideally, the mean of the readings must equal the center of the spec limits); and 2) how widely spread the readings are (ideally, the standard deviation of the readings should be zero). The higher the Cpk, the better is the capability of the process to meet its requirements.
In the industry, a Cpk of less than 1.66 needs a closer look. A Cpk that’s less than 1.33 needs some action to make it higher, and a Cpk of less than 1.0 means that the process is not capable of meeting its requirements.
A low Cpk means one of three things: the mean is far from the center of the specs, or the standard deviation of the readings is high (i.e., the readings are widely spread), or both conditions exist.