Out-of-Control Criteria

The purpose of constructing the control chart is to graphically identify trends in data that indicate areas that must be addressed. When using control charts, safety managers will want to note data that is a trend or data that is out of control. Data that consistently falls between the upper and lower control limits, or data that is in control, may indicate that the control limits need to be recalculated with the more current data, thus tightening the control limits and thereby further improving the expected performance levels. 

There are various criteria that have been established that define a control chart as being out of control. These criteria are (Duncan 1974,392)  

One or more points outside the limits on a control chart. 

One or more points in the vicinity of a warning limit. This suggests the need for immediately taking more data to check on the possibility of the process being out of control. 

A run of 7 or more points. This might be a run up, a run down, or simply a run above or below the central line on e control chart. 

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