Rules for identifying Special Cause Variation (SCV) in run charts
| SCV is detected using the following rules | |
| System shifts | Six or more consecutive points either all above or all below the mean line. |
| Trends | Five or more consecutive increasing or decreasing points. |
| Too many/too few runs | The number of times data crosses the mean line is too many or too few (based on the total number of observations).9 |
| Astronomical points | Data points outside control limits that are obviously different from the rest of the points; all studying the chart would agree the point is unusual. |