A generalized variance chart consists of:

·    Plotted points, each of which represents the generalized variance for each subgroup.

·    A center line (green), which is the median of the theoretical distribution of generalized variance statistics.

·    Control limits (red), which provide a visual means for assessing whether the process is in control. The control limits represent the amount of variation expected for the subgroup generalized variance statistics.

Minitab marks points outside of the control limits with a red symbol, which indicates the presence of special causes. When you append the graph to the ReportPad, the complete test results are available. A failed point indicates that a nonrandom pattern exists in the data that may be the result of special-cause variation. You should investigate these points.

You can summarize the generalized variance chart for the plastics data as follows:

·    The lower and upper control limits are 0 and 6679, respectively. Therefore, you expect the generalized variance statistics to fall between 0 and 6679. The center line, or median, is 668.

·    Sample number 6 falls above the upper control limit.

The generalized variance control chart indicates that the process is relatively in control, except for sample number 6. Try to identify and correct the factors contributing to special-cause variation for sample number 6.