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.
However additional triggers have been proposed that are also sensitive to systematic changes in a process. These are all based on successive values within 1 standard deviation (Zone C), 2 standard deviations (Zone B) or 3 standard deviations (Zone A) from the specified centre.
The five most commonly used indications that a process is out of control are described in the diagram below.
Use the pop-up menu below for a description and example of the five triggers.
The different triggers are illustrated again in the following artificial control chart. The red dots are values that trigger one of the indications that have been described. Try to determine which of the indications have been triggered before clicking on the dot for verification.
Finally choose the option In control data from the pop-up menu to see an example of a typical set of values from a process that is performing according to specifications.