A CUSUM chart with a single two-sided CUSUM consists of the following:

·    Plotted points, which are the CUSUM, the cumulative sum of deviations of the sample values from the target.

·    V-mask, which you can use to assess the process. You can specify the observation on which the V-observation is centered. Center the V-mask on the observation of interest, often the most recent subgroup. If the plotted points fall within the arms of the V, then you can assume that the process is in statistical control.

You should examine the CUSUM charts for

·    Upward or downward trends in the CUSUM. The plotted points should fluctuate randomly around zero. If an upward or downward trend develops, it is evidence that the process mean has shifted and you should look for special causes.

·    Plotted points that are located beyond the V-mask. Points located beyond the V-mask are indicators that the process is out of control. You should initiate a search for potential causes for the special-cause variation.

For the sample data, the V-mask was centered on subgroup 20, because the shift in the process mean occurred after subgroup 20. The CUSUM then begins a downward trend, indicating that the process mean has shifted.