A CUSUM chart with two 1-sided CUSUMs consists of the following:

·    Plotted points (dots), which is the upper CUSUM, the cumulative sum of deviations above the target. The upper CUSUM detects upward trends in the process level.

·    Plotted points (squares), which is the lower CUSUM, the cumulative sum of deviations below the target. The lower CUSUM detects downward trends in the process level.

·    Center line, which is located at zero. The plotted points should fluctuate randomly around the center line.

·    Control limits, which are located (by default) 4 s from the center line, provide a visual means for assessing whether the process is in control. Points located beyond the control limits signal an out-of-control condition.

You should examine the CUSUM charts for

·    Upward or downward trends in the upper and lower CUSUMs. 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 located beyond the control limits, which would indicate that the process is out of control. Minitab displays out-of-control points in red. You should initiate a search for potential causes for the special-cause variation.

For the sample data, the lower CUSUM signals an out-of-control condition from subgroup 24 on. These out-of-control points, which are indicated by the red triangles, fall below the lower control limit. Thus, a search for special causes should ensue.