The between/within capability section includes capability indices associated with the B/W (between and within) 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 of the process 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 relative to the specification interval. Cp 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 mean 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.

·    Note that 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 the specification limits. If Cp is greater than Cpk, then the process is not centered.

A substantial difference between the overall and B/W capability indices may indicate that the process is out of control. For the coating data, the B/W and overall capability indices are very close to each other.

For the coating data, Cp is 1.26, which indicates that the specification spread is 1.26 times greater than the 6-s spread in the process.

Cp (1.26) and Cpk (1.21) are very close to one another, indicating that the process is approximately centered. The differences are due to the mean being slightly below the target value.