Table 1

Rules for identifying Special Cause Variation (SCV) in run charts

SCV is detected using the following rules
System shiftsSix or more consecutive points either all above or all below the mean line.
TrendsFive or more consecutive increasing or decreasing points.
Too many/too few runsThe number of times data crosses the mean line is too many or too few (based on the total number of observations).9
Astronomical pointsData points outside control limits that are obviously different from the rest of the points; all studying the chart would agree the point is unusual.