Multi-process capability plot and fuzzy inference evaluation

Abstract Process capability indices Cp, Cpk, Cpm and Cpp fitting for nominal-the-best type quality characteristics, are effective tools to assess process capability since these indices can reflect a centering process capability and process yield adequately. The index Cpp introduced by Greenwith and Jahr-Schaffrath [Greenwith, M., Jahr-Schaffrath, B.L., 1995. A process incapability index. International Journal of Quality and Reliability Management 12, 58–71] provides additional and individual information concerning the process accuracy and the process precision. Although Cpp is useful to evaluate process capability for a single product in common situation, Cpp cannot be applied to evaluate the multi-process capability. Referring to Vannman and Deleryd's (Cdr, Cdp)-plot, a fuzzy inference approach is proposed in our study to evaluate the multi-process capability based on distance values of a confidence box. This method takes the advantages of fuzzy systems such that a grade instead of sharp evaluation result can be obtained. An illustrated example of color STN display demonstrates that the presented method is effective for assessment of multi-process capability.