The tests for special causes assess whether the plotted points are randomly distributed within the control limits.
Use the tests for special causes to determine which observations you may need to investigate and to identify specific patterns and trends in your data. Each of the tests for special causes detects a specific pattern or trend in your data, which reveals a different aspect of process instability. For example, Test 1 detects a single out-of-control point. Test 2 detects a possible shift in the process.
For the traditional control charts for variables data, Tests 1, 5, 6, 7, and 8 are based on the normal distribution. However, for T charts, these tests are based on the Weibull distribution or the exponential distribution. For example, a point on a T chart fails Test 1 when the point is outside of the percentile range that correspond to 3 standard deviations from the mean for a normal distribution.
Eight tests are available with this control chart.