Department of Statistics National Chengchi University
In this paper, we propose an efficient approach to distinguish which process step is out of control and examine which kind of special cause occurs the shift of process means for two-grade products produced from two dependent processes. Two EWMA control charts are proposed to detect if the first kind of special causes, which affect one of the two-grade products, occur; and a combined EWMA control chart is designed to examine if another kind of special causes, which affect both the two-grade products, occur in the previous process. Two EWMA-type cause-selecting control charts are proposed to examine if the first kind of special causes, which affect the interested quality characteristic for each grade products, occur in the previous process. A combined cause-selecting control chart is designed to detect if another kind of special causes, which affect both the interested quality characteristics for two-grade products, occur in the current process. A simulated data analysis shows our proposed control charts are superior to Shewhart control charts and Hotelling T control charts in distinguishing which process is out of control and detecting which kind of special causes occur for two-grade products.
Keywords: control chart, special cause, dependent processes
Cite this article as: Su-Fen Yang and Tar-Hsing Yang, "Statistical Process Control for Two Grade Products under Two Dependent Processes," Journal of the Chinese Institute of Industrial Engineers, 18, 25-44 (2001).