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G ChartStat > Control Charts > Rare Event Charts > G
Use a G chart to monitor the number of opportunities or, in many cases, the number of days between rare events, such as infections or surgical complications.
When you monitor rare events with a traditional attribute chart, such as a P or a U chart, you need a large amount of data to establish accurate control limits. Therefore, collecting enough rare event data to detect an adverse change in the frequency of events may take months or even years. You do not need to collect large amounts of data to use a G chart. The G chart is based on the geometric distribution. The G chart plots the number of opportunities between events so you can easily detect when events occur more or less often than expected.
In some situations, the actual number of opportunities between events is not known. Instead, the dates of the events are recorded. When you use the G Chart in these situations, you should verify that the number of possible opportunities per day is fairly constant.
If you know the date and specific time of day for each event, use the T chart to monitor time between rare events.
For more information, see Control Charts Overview and Rare Events Charts Overview.
Form of data:
Dates of event: Choose if your data are recorded as the date of each event.
Number of opportunities: Choose if your data are recorded as the number of opportunities between or until an event.
Method of counting opportunities:
Opportunities between events (0, 1, 2, 3, ...): Choose if your data are recorded as the number of opportunities between events. For example, if events occur on the 5th and 8th opportunities, the number of opportunities between events is 2 because 6 and 7 are the opportunities between 5 and 8.
Opportunities until an event (1, 2, 3, 4, ...): Choose if your data are recorded as the number of opportunities until an event occurs. For example, if events occur on the 5th and 8th opportunities, the number of opportunities until an event is 3 because 6, 7, and 8 are the opportunities until the event occurs.
Variables: Enter one or more columns that contain the rare event data.