Quality Advisor


What is it?

A g-chart is a chart for attributes data. It is used to count the number of events between rarely-occurring errors or nonconforming incidents.

The g-chart creates a picture of a process over time. Each point represents the number of units between occurrences of a relatively rare event. For example, in a production setting, where cars are produced daily, an unexpected line shutdown may occur. A g-chart might be used to look at the number of units (i.e. cars) produced between line shutdowns. The units produced can be almost anything. For example, you might look at the number of invoices printed, the number of customers served, or the number of patients seen, between occurrences of some event. A traditional plot of data such as this is not conducive to control chart interpretation. The g-chart helps to visualize this data in traditional control chart form. Specific formulas for g-chart control limits are used with this type of data.

The “g” in g-chart stands for geometric, since data relating to events between occurrences represents a geometric distribution. G-charts can be created using software programs like SQCpack.

What does it look like?


When is it used?

A rare event chart is used when a traditional control chart is not effective. Consider a rare event chart when one or more of these conditions exist:

  1. More than 20% of the data being counted has a numerator of zero.
  2. The denominator of a typical count-based control chart is so large that the control limits are excessively close to the average.
  3. The LCL (lower control limit) is missing.


Why is a t-chart used?

  • To see if the process is stable and predictable
  • To see how a planned change affects the process
  • To track the passage of time between relatively rare events


Why is a g-chart used?

  • To see if the process is stable and predictable
  • To see how a planned change affects the process
  • To track the number of units counted between relatively rare events


Getting the most

Here are some processes in various industries that lend themselves to rare event application.



  • Wrong-site surgeries
  • Patient falls
  • Infection outbreaks
  • Accidental needle sticks
  • Harmful medication errors




  • Product liability lawsuits over 1 million dollars
  • Accidental deaths or dismemberments
  • Production line stoppages
  • Days since last accident




  • Power outages over 1 minute
  • Bank statement errors




  • School closings
  • Days between a student’s absences