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.