Comparison of two process capabilities by using indices C pm : 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 C p and C pk failed to account for process centering, index C pm was developed, which considers process centering and is suitable for processes of the nominal‐the‐best type. Other indices like C pu and C pl also exist, and are used for unilateral specification. Chou developed a procedure using estimators of C p , C pu and C pl to allow practitioners to determine whether two processes are equally capable. For bilateral specification processes, index C p fails to measure process yield and process centering, and thus the index C pm 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.