where d = (USL LSL)/2 is half width of the specification interval and M = (USL + LSL)/2 is mid-point of specification limits.
Boyles developed a relationship between the bounds on process yield and the index Cpk. But in practice, the population
mean and variance are usually unknown and needed to be further estimated on the basis of the sample information. The
natural estimators of l and r2 are X and S2, respectively, where X is the sample mean and S2 is the sample variance. Therefore, the natural estimators of Cpk, C^ pk, can be expressed as