Capability indices for non-normal distributions with an application in electrolytic capacitor manufacturing

Process capability indices C-p(u, v), which include the four basic indices C-p, C-pk, C-pm and C-pmk as special cases, have been proposed to measure process potential and performance. C-p(u,v) are appropriate indices for processes with normal distributions, but have been shown to be inappropriate for processes with non-normal distributions. In this paper, we first consider two generalizations of C-p(u, v), which we refer to as C-Np(u, v) and C-Np(u, v), to cover cases where the underlying distributions may not be normal. Comparisons between C-Np(u, v) and C-Np'(u, v) are provided. The results indicated that the generalizations C-Np(u, v) are superior to C-Np'(u, v) in measuring process capability. We then present a case study on an aluminum electrolytic-capacitor manufacturing process to illustrate how the generalizations C-Np(u, v) may be applied to actual data collected from the factories. (C) 1997 Elsevier Science Ltd.