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T ChartStat > Control Charts > Rare Event Charts > T
Use a T chart to monitor the time between rare events, such as infections or surgical complications.
When you monitor rare events with a traditional variables control chart, you need a large amount of data to establish accurate control limits. Therefore, collecting enough rare event data to respond to an adverse change in the frequency of events may take months or even years. You do not need to collect large amounts of data to use a T chart. A T chart plots the time between events so you can easily detect when events occur more or less frequently than usual.
By default, Minitab bases the calculations for a T chart on the Weibull distribution. However, you can also choose to base the calculations on the exponential distribution. (See T Chart Options > Parameters or Tools > Options > Control Charts and Quality Tools > Other.)
If you know the date but not the specific time for each event, use the G chart to monitor the number of complete days between rare events. For more information, see Control Charts Overview and Rare Events Charts Overview.
Form of data:
Dates/Times of events or Time between events: Choose if your data are recorded as the date and time of each event or the amount of time between events (elapsed time).
Number of intervals between events: Choose if your data are recorded as the number of intervals between events.
Units for intervals:
Days: Choose if your data are recorded as the number of days between events. For example, a value of 4.5 indicates 4 and a half days between events.
Hours: Choose if your data are recorded as the number of hours between events. For example, a value of 12.25 indicates 12 hours and 15 minutes between events.
Variables: Enter one or more columns that contain the rare event data.