Determining the Time of a Permanent Shift in the Process Mean of CUSUM Control Charts

Abstract

Control charts are used widely to distinguish between random and assignable causes of variation in the process, i.e., they are used to monitor for permanent shifts in the process. However, a control chart does not indicate the exact time when such a shift occurs. Since a shift in the process usually occurs much earlier before it is detected, the search for the assignable cause could be quite difficult. The search depends on the process engineers’ experience and knowledge of the process. This article discusses an approach based on the work of Samuel et al. (see Samuel, T. R., Pignatiello, J. J. Jr., Calvin, J. A. (1998). Identifying the Time of a Step Change with [Xbar] Control Charts. Quality Engineering, 10(3):521–527.) to identify the time of a permanent shift in the mean of a process with CUSUM control charts.