G charts monitor the number of opportunities or, in many cases, days between rare events. The tests for special causes detect points beyond the control limits and specific patterns in the data plotted on the chart.

·   When a point fails a test, Minitab reports it in the Session window and marks it on the G chart. A failed point indicates that there is a nonrandom pattern in the data that may be the result of special cause variation. These points should be investigated.

·   If no points fail the test for special causes, then no information is displayed in the Session window.

Observations 30 and 31 failed the Benneyan test (3 points in a row are equal to 0), which indicates that 3 or more infections were recorded on the same day. The data analyst examines the worksheet and observes that hospital staff recorded 5 infections on October 26.