MINI PAPER - ASQ

16 ASQ STATISTICS DIVISION NEWSLETTER, VOL. 18, NO. 1 Process capability indices have been popular for over20 years, since Joseph Juran12popularized the CapabilityRatio (Cr) in his Quality Control Handbook. Eagerness toestablish a single index to measure process capability hasresulted 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 welldocumented (Gunter)8. Though simple to compute, theycan lead to scorekeeping by management, incorrect inter-pretations and tampering, with little or no product orprocess improvement. This article reviews the basics ofestimating process capability and introduces new processcapability , CpkThe original Capability Index published in Juran sQuality Control Handbookis defined as Tolerance Widthdivided by Process = Process Potential = (Tolerance Width) / (Process Capability).

18 ASQ STATISTICS DIVISION NEWSLETTER, VOL. 18, NO. 1 Continued on page 19 MINI-PAPER Continued from page 17 %OOL Percent Outside of Limit uses the z-statistics to esti- mate the porportion of a population that lies beyond the spec limits.

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