Process Capability Evaluation for a Non-normally Distributed One

Abstract

The common process capability indices (PCIs) Cp, Cpk, Cpm are widely used inpractice. The use of these PCIs is based on the assumptions that process is incontrol and its output should be normally distributed. In practice normality is notalways fulfilled. Therefore, the use of common PCIs leads to erroneous incapability evaluation. In this paper, capability evaluation for non-normallydistributed process is carried out in industrial environment with two approaches.The first includes transforming data to normally distribute by Box-Coxtransforming method then using the common PCIs. This method failed to transformthese data. The second approach includes the use non-normal percentile methodwith Burr XII distribution. This paper proves that the second approach is moreeffective in evaluating the capability of this process. Practical case is applied in theState Company for Electrical Industries (SCFEI) particularly in (Water Pump)factory and Minitab 16 Software is used to reduce the long calculation of statisticalvalues and to plot control charts.