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A rare event chart is a control chart that provides information about the process being studied where the data comes from rarely-occurring incidents. It provides insight into a process when tracking quality measures that occur infrequently enough that tracking them on a traditional control chart tends to be ineffective. Rare event charts were developed in response to the limitations of attributes control charts and individuals charts in rare event scenarios.
In a typical count or attributes control chart, there is a numerator (what is counted) and a denominator (the sample size or items in the pool to count). If many of the periods being counted are zero because no incident has occurred, a chart with many flat lines ensues, where everything is plotted along the x-axis. When an incident occurs, the count of one for this period is plotted, then the counts drop back to zero for the next few periods. This cycle continues, and the resulting control chart looks odd, the control limits are not effective, and the chart fails to provide proof of quality performance.
Two types of rare event charts exist. A t-chart measures the time elapsed since the last event, and a g-chart counts the number of possible opportunities since the last event.