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Publication date: 07/30/2020

Rare Event Control Charts

A Rare Event chart is a control chart that provides information about a process where the data comes from rarely occurring events. Tracking processes that occur infrequently on a traditional control chart tend to be ineffective. Rare event charts were developed in response to the limitations of control charts in rare event scenarios. Control Chart Builder provides two types of rare event charts (G and T charts). The difference between a G and T chart is the scale used to measure distance between events. The G chart uses a discrete scale, whereas the T chart uses a continuous scale.

Table 3.2 Rare Event Chart Determination Based on Sigma

Distribution Used to Calculate Sigma

Statistic Type: Count

Negative Binomial

G chart


T chart

G charts

A G chart is used to count the number of events between rarely occurring errors or nonconforming incidents, and creates a chart of a process over time. Each point on the chart represents the number of units between occurrences of a relatively rare event. For example, in a production setting, where an item is produced daily, an unexpected line shutdown can occur. You can use a G chart to look at the number of units produced between line shutdowns.

T charts

A T chart measures the time elapsed since the last event. Each point on the chart represents an amount of time that has passed since a prior occurrence of a rare event. A T chart can be used to numeric, nonnegative data, date/time data, and time-between data. Since a traditional plot of this data might contain many points at zero and an occasional point at one, using a T chart avoids flagging numerous points as out of control.

When reading a T chart, the points above the upper control limit indicate that the amount of time between events has increased. This means that the rate of adverse events has decreased. Therefore, a point flagged as out of control above the limits is generally considered a desirable effect when working with T charts.

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