Use generalized variance charts to determine whether or not the joint process variability for two of more variables is in control.

·    An in-control process exhibits only random variation with the control limits.

·    An out-of-control process exhibits unusual variation, which maybe due to the presence of special causes.

Generalized variance charts allow you to simultaneously monitor whether the joint variability of two or more related variables is in control.

A plastics company wants to track the process variability of tear strength, glossiness, and opacity of sheets of plastic. You collect 4 samples over 10 days. These variables are correlated, so you decide to create a generalized variance control chart to monitor the joint variability.

Data: PlasticSheets.MTW (available in the Sample Data folder).