Note: Use this test for control sparingly. There is a tendency to overcontrol the system when using this test. Use it only when there is some doubt about the system’s stability.
The chart below shows a typical sawtooth pattern. Observe how the data points alternate above and below the center line. For some reason, alternate subgroups have greater and smaller averages. Stratifying or splitting the data by key variables may assist in analyzing this problem. This may occur if you alternate samples from two machines or production lines.