Measuring process yield based on the capability index Cpm.

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=2F(3C_pk)-1=2F(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. [ABSTRACT FROM AUTHOR]