The potential (within) capability indices are associated with the within-subgroup standard deviation.

The industry guidelines determine whether the process is capable. A generally accepted minimum value for the indices is 1.33.

These indices reflect how the process could perform relative to the specification limits, if the shifts and drifts between subgroups could be eliminated. The capability indices consist of the following:

·    Cp - relates the process spread (the 6-s variation) to the specification spread. In other words, Cp relates how the process is performing to how it should be performing. Cp does not consider the location of the process mean, so it tells you what capability your process could achieve if centered.

·    CPU and CPL - relates the process spread (the 3-s variation) to a single-sided specification spread (m-LSL or USL-m). CPL and CPU consider both process center and process spread. Use CPL and CPU when you have a single-sided specification limit. For example, if you want a cable to meet a minimum strength, you might compare the process to the lower specification limit.

·    Cpk - Minimum of CPU and CPL. Cpk incorporates information about both the process spread and the process mean, so it is a measure of how the process is actually performing. Cpk considers the location of the process mean, while Cp does not. If Cp and Cpk are approximately equal, then the process is centered between specification limits. If Cp is greater than Cpk, then the process is not centered.

A substantial difference between the overall and within capability indices may indicate that the process is out of control, or the sources of variation are not estimated by the within-subgroup component of variation. For the piston data, the within and overall capability indices are very close to each other.

For the piston data, Cp is 1.66, which indicates that the specification spread is 1.66 times greater than the 6-s spread in the process.

Cp (1.66) and Cpk (1.62) are very close to one another, indicating that the process is centered on target. The capability indices are greater than 1.33, indicating that the process is centered on target and capable of producing pistons that conform to specifications.