The Taguchi Capability Index

The process capability indices Cp and Cpk 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 Cp advocated by Taguchi addresses this issue directly. Later authors introduced the name Cpm 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. Cpm and Cpk are compared and contrasted to dispel the notion that Cpk measures process centering.