The Taguchi Capability Index

Abstract

The process capability indices C_p and C_pk are widely used to provide unitless measures of process potential and performance. These indices do not adequately address the issue of process centering. An alternative definition of C_p advocated by Taguchi addresses this issue directly. Later authors introduced the name C_pm for the Taguchi index and examined statistical properties of an inefficient estimator under the assumption that the process mean coincides with the target value. The present paper presents statistical procedures based on the original Taguchi estimator which require no assumptions on the process mean. C_pm and C_pk are compared and contrasted to dispel the notion that C_pk measures process centering.