Print ISSN: 1681-6900

Online ISSN: 2412-0758

Author : Abbas Mahmoud, Mahmoud


Process Capability Evaluation for a Non-normally Distributed One

Samah Ali Aufy; Mahmoud Abbas Mahmoud

Engineering and Technology Journal, 2013, Volume 31, Issue 17, Pages 3245-3258

The common process capability indices (PCIs) Cp, Cpk, Cpm are widely used in
practice. The use of these PCIs is based on the assumptions that process is in
control and its output should be normally distributed. In practice normality is not
always fulfilled. Therefore, the use of common PCIs leads to erroneous in
capability evaluation. In this paper, capability evaluation for non-normally
distributed process is carried out in industrial environment with two approaches.
The first includes transforming data to normally distribute by Box-Cox
transforming method then using the common PCIs. This method failed to transform
these data. The second approach includes the use non-normal percentile method
with Burr XII distribution. This paper proves that the second approach is more
effective in evaluating the capability of this process. Practical case is applied in the
State Company for Electrical Industries (SCFEI) particularly in (Water Pump)
factory and Minitab 16 Software is used to reduce the long calculation of statistical
values and to plot control charts.