A new process capability index for non-normal distributions

Many process capability indices have been proposed to measure process performance. In this paper, we first review C_p, C_pk, C_pm and C_pmk, and their generalizations, C_Np, C_Npk, C_Npm and C_Npmk, and then propose a new index S_pmk for any underlying distribution, which takes into account process variability, departure of the process mean from the target value, and proportion of nonconformity. Proportion of nonconformity can be exactly reflected by S_pmk. Its superiority over C_Npmk, a recently developed index, also taking into account process variability and departure from the target value, is demonstrated with several non-normal processes. A method is proposed to estimate S_pmk, with illustrations.