Identifying the time of a step change with X control charts

摘要: Control charts are used to monitor for changes in a process by distinguishing between common and special causes of variability (1). When a control chart signals' that a special cause is present, process engineers must initiate a search for the special cause of the process disturbance. The search depends on the process engineers' expemse and knowledge of the process. Identifying which combination of the many process variables is responsible for a change in the process allows engineers to improve quality by preventing or avoiding changes in those variables which lead to poor quality, and by perpetuating those changes and optimizing those variables which can lead to better quality.Knowing when a process has changed would simplify the search for the special cause. If the time of the change could be determined, process engineers would have a smaller search window within Which to look for the special …