Rare Events Control Charts Overview  
      

Some events occur so infrequently that you cannot use a traditional control chart, such as a Xbar-R or P chart, to monitor your data. Examples of rare events include hospital-acquired infections, medication errors, or manufacturing processes which have a low defect rate.  

Control charts for rare events show the amount of time or the number of opportunities between events. Plotted points that are higher on a control chart for rare events indicate a longer time between events. Plotted points that are lower on the chart indicate less time between events.

Minitab provides the following control charts to monitor rare events:

·    G Chart: Use a G chart when the data are either the date of the event or the number of opportunities before or until the event occurs.

·    T Chart: Use a T chart when the data are either the date and time of the events, the elapsed time between events, or the number of intervals between events.