QUALITY AND RELIABILITY CORNER Comparison of two process capabilities by using indices Cpm: an application to a color STN display

Recent years have seen mounting interest in measuring process performance in the manufacturing industry. Analysis of process capability indices allows a production department to trace and improve a poor process to enhance quality and satisfy customers. Process capability analysis can also serve as an important reference in determining strategies to improve global product quality. Since Cp and Cpk failed to account for process centering, index Cpm was developed, which considers process centering and is suitable for processes of the nominal-the-best type. Other indices like Cpu and Cpl also exist, and are used for unilateral specification. Chou developed a procedure using estimators of Cp, Cpu and Cpl to allow practitioners to determine whether two processes are equally capable. For bilateral specification processes, index Cp fails to measure process yield and process centering, and thus the index Cpm is used to develop a similar procedure to help practitioners determine whether or not two processes are equally capable. Naturally, decisions made using the procedure to select the better supplier are more reliable than decisions made using other methods.