Process capability analysis is designed to estimate the proportion of parts that do not meet engineering requirements in a stable production process.When the quality of products is related to two or more variables, the analysis should be based on a multivariate statistical technique. In this paper, we review and criticize the capability indices typically used in industry for this purpose, and propose a general multivariate process capability index based on fuzzy tolerance region which has not the restriction of conventional process capability indices. We further derive its analytical expression under standard assumptions, discuss numerical approximations, and compare three traditional process capability indices with the new index based on fuzzy approach.

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