Additional triggers for an out-of-control process
The most commonly used indication of a process being out-of-control is a value outside the upper and lower control limits (more than three standard deviations away from the centre line). This is sensitive to changes to the process mean or increases in the process variability.
Additional triggers have been proposed that are also sensitive to systematic changes in a process. These are all based on successive values within 1, 2 or 3 standard deviations from the centre (Zones C, B or A).