Complete the following steps to interpret a CUSUM chart. Key output includes the CUSUM chart and test results.

The CUSUM chart plots the cumulative sums (CUSUMs) of the deviations of each sample value from the target value. Because the CUSUM chart is cumulative, even minor drifting in the process mean will cause steadily increasing (or decreasing) cumulative deviation values.

You can create either a tabular CUSUM or a V-mask CUSUM.

A CUSUM chart with two tabular CUSUMs consists of the following:

- Plotted points (dots), which are the cumulative sum of deviations above the target.
- Plotted points (diamonds), which are the cumulative sum of deviations below the target.
- Center line, which is located at zero.
- Control limits, which are located 4 standard deviations from the center line.

On a tabular CUSUM chart, look for the following:

- 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, the process mean has shifted and the process may be affected by special causes.
- Plotted points that are located beyond the control limits, which indicates that the process is out of control.

A CUSUM chart with a single V-mask consists of the following:

- Plotted points, which are the cumulative sum of deviations of the sample values from the target.
- V-mask, which is used instead of control limits to determine out-of-control points. The V-mask standardizes the deviations from the target value, and plots the deviations from this value.

On a V-mask CUSUM chart, look for the following:

- Upward or downward trends in the CUSUMs. If an upward or downward trend develops, the process mean has shifted and the process may be affected by special causes.
- Plotted points that are located beyond the V-mask, which indicates that the process is out of control.

When you create a CUSUM chart, a table displays which points are beyond the control limits. Investigate any subgroups that are outside of the control limits.

TEST. One point beyond control limits.
Test Failed at points: 24, 25, 26, 27, 28, 29, 30