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A cumulative sum (CUSUM) chart is a type of control chart used to monitor small shifts in the process mean. It uses the cumulative sum of deviations from a target.
The CUSUM chart plots the cumulative sum of deviations from the target for individual
measurements or subgroup means.
It is possible to modify the CUSUM, so it responds more quickly to detect a process that is out-of-control at start-up. This modification is done using an initial CUSUM equal to half of the h parameter.
Like the EWMA, CUSUM is sensitive to small shifts in the process mean but does not match the ability of a Shewhart chart to detect larger shifts. For this reason, it is sometimes used together with a Shewhart chart (Montgomery 2012).