Multivariate control charts, such as Hotelling's T2 control chart, are used for monitoring several quality characteristics measured simultaneously on a product or process. In the retrospective stage (Phase I) analysis of a historical data set, the presence of multiple outliers may go undetected by the conventional T2 chart, due to masking. In this paper we propose T2 control charts based on robust estimators of location and dispersion. Simulation studies show that the T2 control chart using the minimum volume ellipsoid (MVE) estimators is effective in detecting any reasonable number of outliers.
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