Hotelling's T2 is customarily used as the control chart for multivariate SPC analysis. This chart responds to changes in both the mean values and the covariance matrix of the responses. In this article, we propose the use of a chart that concentrates on changes in the covariance matrix. The use of this covariance chart in concert with the T2 chart enables the user to better determine whether T2 points out of control are due to changes in mean values or due to changes in the covariance matrix. Using this chart in conjunction with T2 thus furnishes a suite of tools similar to the x-bar and standard deviation charts for univariate processes.
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Quality Engineering. 2000. 14. (4). 539-545.
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