G charts monitor the number of opportunities or, in many cases, days between rare events. The G chart consists of the following elements:

·   Plotted points, which represent either the number of days between events or the number of opportunities between events, depending on the data in your worksheet.

·   Center line (green), which is the 50th percentile of distribution.

·   Control limits (red), provide a visual way to assess when to take action on the process. In a G chart, the lower control limit is always set to 0.

For G charts, Minitab conducts up to five tests for special causes, each of which detects a specific pattern in the data. When a data point fails a test, Minitab marks it on the G chart with a red symbol and with the number or letter of the test that failed. A failed point indicates that there is a nonrandom pattern in the data which may be the result of special cause variation. These points should be investigated.

The G chart for the infection data can be summarized as follows:

·    The lower and upper control limits are 0 and 62.10, respectively. Therefore, the number of days between infections is expected to be between 0 and 62. The center line is 5.63.

·    Two points 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 event probability is 0.099. The event probability is the chance of an event occurring on any given day. For the infection data, the chance of an infection occurring on a given day is 9.9%.

Next, you should try to identify and correct the factors that contribute to this special cause variation. Until these causes are eliminated, the process cannot achieve a state of statistical control.