Abstract
AbstractA standard process capability index is calculated based on the assumption that the quality characteristic of the process follows the normal distribution. But there are many cases in which the quality characteristic comes from a non-normal distribution. This paper studies Box-Cox transformation method and Weighted Variance method to calculate process capability indices for Weibull distributed quality characteristic and compares performances of these methods. Weibull distribution is extensively used as a lifetime distribution model because of its flexible shape. The data sets used in performance comparison are randomly generated from Weibull distribution for two different shape and scale parameters through a simulation study. The results indicate that Box-Cox transformation method produces better estimates for process capability than Weighted Variance method.
Highlights
Process capability is a performance measure to compare process variation with the product specifications
Process capability indices (PCIs) are widely used in industry to measure the ability of the process of the firm or its supplier to manufacture product that meets quality specifications
There are many cases in which the quality characteristic comes from a non-normal distribution
Summary
Process capability is a performance measure to compare process variation with the product specifications. Process capability indices (PCIs) are widely used in industry to measure the ability of the process of the firm or its supplier to manufacture product that meets quality specifications. Several PCIs including Cp, Cpu, Cpl, Cpk, and Cpm (Equation (1)) have been used in the manufacturing industry to provide common quantitative measures on process potential and performance [1]. USL LSL Cp USL P Cpu P LSL Cpl (1) Cpk min® ̄ USL 3V P
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