Another type of capability index, Cpm, is much less known and used less frequently than Cpk or Ppk. The main difference between the Cpm and the other capability indices is that the bias from the target, not mean, is directly taken into consideration in the Cpm. The bias is the difference between the process average and the target.
Cpm can be calculated by the above equation(reference 1). Compared with Cpk or Ppk, Cpm is quite effective to avoid mismatch issues and for asymmetrical targets. Details can be obtained in reference 2.