ASQ - Statistics Division

Process Capability Indices – Mini Paper Appearing in Winter 1999 Newsletter

Abstract: Process capability indices have been popular for over 20 years, since Joseph Juran popularized the Capability Ratio (Cr) in his Quality Control Handbook. Eagerness to establish a single index to measure process capability has resulted in the proliferation of indices: Cp/Cpk, Vp/Vpk, Pp/Ppk, Cpm, Tz, %OOL, PPM, and more recently – “Generalized” Cp/Cpk. Abuse of these indices is well documented. Though simple to compute, they can lead to scorekeeping by management, incorrect interpretations and tampering, with little or no product or process improvement. This article reviews the basics of estimating process capability and introduces new process capability indices.

Keywords: Process capability - X-bar and R chart - Special cause - Common cause - Variability

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