Measuring process yield based on the capability index C-pm

Abstract

Process capability indices C-p, C-a, C-pk and C-pm have been proposed to the manufacturing industry as capability measures based on various criteria including variation, departure, yield, and loss. It has been noted in recent quality research and capability analysis literature that both the C-pk and C-pm indices provide the same lower bounds on process yield, that is, Yield greater than or equal to 2Phi(3C(pk)) - 1 = 2Phi(3C(pm)) - 1. In this paper, we investigate the behaviour of the actual process yield in terms of the number of nonconformities (in ppm) for processes with a fixed index value of C-pk = C-pm, but with different degrees of process centring, which can be expressed as a function of the capability index C-a. The results illustrate that it is advantageous to use the index C-pm over the index C-pk when measuring process capability, since C-pm provides better customer protection.