Cusum control charts are used when it is important to detect that a process has wandered away from a specified process mean. Although Shewhart
-charts can detect if a process is moving beyond a two- or three-sigma shift, they are not effective at spotting a one-sigma shift in the mean. They still appear in control because the cumulative sum of the deviations wanders farther away from the specified target. A small shift in the mean also appears very clearly and much sooner.