g: Number of Non-events Between Events

When monitoring a process that happens infrequently or rarely, a normal SPC chart may not be very useful - as most values would be zero. Instead of monitoring a monthly proportion/rate of events, we can instead monitor the number of times that an event did not occur (when it had the opportunity to) before an event is recorded.

The g Chart below monitors the number of successful surgeries between each recorded fatality.

To construct this chart, you provide the outcome variable of interest (number of non-events) in the ‘Outcome/Numerator’ field, and the date of each event in the ‘ID Key’ field, with one row per event (depicted below). The x-axis shows the event number rather than the date. Then, select g - Number of Non-Events Between Events from the Chart Type drop-down in Data Settings.