The zones in an ordinary 3-sigma control chart are beyond the upper control limit (3,∞), and below the lower control limit (-∞, -3).
- To specify the alarm zone as the area between the warning limits and control limits, enter a=2, b=3.
- In a control chart with 0.001 probability control limits (3.09 "sigma") and 0.025 warning limits (2.24 "sigma"), the rule "two consecutive points between the control and warning limits" is given by: k=2, a=2.24, b=3.09.
- The zones are usually symmetric around the center line. For example: [-3,-2] and [2,3].