Control charts for rare events – T charts
Rare event charts are used to plot intervals between events. Time between events tends to follow a Weibull distribution. The T chart is used to track whether the intervals between events are changing or remain stable.
If we were to plot the number of events in a day or week, it may be possible to use this data as a c-chart that displays a count of events. The problem with this technique is that rare events tend to have a rate that is very close to zero. Any event that occurs will be flagged up as unusually high.
Here we use the T chart to plot the time between eruptions of Vesuvius. Vesuvius is historically a very active and dangerous volcano, which was responsible for the destruction of Pompeii and Herculaneum ...
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