2 of 3 beyond 2 sigma
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 control limits drawn on control charts are located three standard deviations away from the average (or center line) of the chart. These are called “3-sigma” control limits. Sigma is the name of the symbol for standard deviation. The distance from the center line to the control limits can be divided into three equal parts, one sigma each, as shown below. If two out of three consecutive points on the same side of the average lie beyond the 2-sigma limits, the system is said to be unstable. The chart below demonstrates this rule.