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B: Monitoring Rare Events with G Charts and T Charts (Control Charts)

How can I monitor rare events (nonconformances, rare incidents, deviations) that are critical to quality but occur infrequently?










Solution 1: G chart (Geometric distribution)
The G chart is a control chart designed for monitoring rare events that are identifed by attributes data. These type of charts are typically used to measure the number of days between the rare events. Another possibility is to use the G chart to measure the number of opportunities between rare events.

The “g” in g-chart stands for “geometric” because data relating to normal events between rare events represents a geometric distribution.

Solution 2: T chart
The T chart is a control chart designed for monitoring rare events but analyzes time data (e.g. hours, days, weeks, months between the rare events).