A comparative study of joint monitoring schemes for APC processes

To monitor processes under automatic process control (APC), several joint monitoring schemes have recently been proposed, such as the U statistics, U ₀ and U_∞, that originate from the uniformly most powerful test. It is known that the U₀ chart is sensitive to large shift detection whereas the U_∞, chart is sensitive to small shift detection. To take advantage of both charts, this paper examines a combined use of the two U charts. A comparative study indicates that the combined procedure improves over a single U chart and performs better than other joint monitoring schemes, including Hotelling's T² chart for APC processes.