A note on the conservative nature of the tables of lower confidence limits for c_pk with a suggested correction

Abstract

Chou, Owen and Borrego A. provided tables of 95% lower confidence limits for the process indices C_pCPU (CPL), and C_pk. This represented a major step toward the proper use and understanding of such process indices. However, the simulations in this paper indicate that their tables for C_pk can produce 97 or 98% lower confidence limits (instead of 95%) making them conservative. An approximation suggested by Bissell is shown to provide the desired 95% lower confidence limits for C_pk for n > 30.

Keywords:

• Process indices
• 95% lower confidence limits
• simulation