Publication date: 05/24/2021

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 charts and T charts). The difference between a G chart and a T chart is the quantity used to measure distance between rare events. The G chart measures counts of events between incidents, whereas the T chart measures time intervals between incidents.

Table 3.2 Rare Event Chart Determination Based on Sigma

Distribution Used to Calculate Sigma

Chart Type

Negative Binomial

G chart

Weibull

T chart

G charts

A G chart measures 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.

When reading a G chart, the points above the upper control limit indicate that the number of events between errors has increased. If the number of events between rarely occurring errors or nonconforming incidents has increased, that is good. Therefore, a point flagged as out of control above the limits is generally considered a desirable effect when working with G charts.

T charts

A T chart measures the time intervals elapsed since the last event. Each point on the chart represents a number of time intervals that have passed since a prior occurrence of a rare event. A T chart can be used for numeric, nonnegative data, date/time data, and time-between data. Since a traditional plot of these 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. The data points for a T chart in Control Chart Builder are restricted to integer values.

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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