The CUSUM chart plots cumulative sums in each direction (positive and negative) independently.
“CUSUM (Cumulative Sum) charts improve the ability to detect small shifts (i.e. less than 1.5 σ) by charting a statistic that incorporates current and previous data values from the process. Specifically, the CUSUM chart plots the cumulative sums of the deviations of the sample values from a target value. The inclusion of several samples in the
cumulative sum results in greater sensitivity for detecting shifts or trends over the traditional Shewhart charts (Individuals or Xbar charts).”