Volume 38, Issue 2 (March 2010)
Process Performance Evaluation with Imprecise Information
Process capability indexes have been widely used in the manufacturing industry for measuring process performance. Most studies on assessing the process capability are based on crisp estimates of process capability indexes while assuming output measurements of the process being precise number. However, in practical situations it is much more realistic that the output characteristics of continuous quantities are more or less imprecise in general. The fuzziness and stochastic uncertainty are unavoidable features of collected observations from continuous quantities. To assess the process performance more accurately, we consider the capability index CPMK, which is recognized as the most useful index to date for assessing the process capability with two-sided specification limits. We construct the fuzzy estimator for the index CPMK according to the fuzzy set theory. To make reliable decisions, we propose a decision rule to assess the process capability based on the fuzzy lower confidence bound of CPMK. A step-by-step procedure for testing process capability is provided for easy execution in practical situations. An example of manufacturing silicon wafers is also presented to illustrate the applicability of the proposed approach.