ABSTRACT
Multivariate control methods, although originating with the work of Hotelling, have only recently become an active and fruitful field or research. The use of Process capability indices (PCIs) in connection with multivariate measurements is hedged around with even more cautions and drawbacks than is the case for univariate measurements. In fact, most of the multivariate PCIs which have been proposed, can be best understood as being univariate PCIs, based on a particular function of the variables representing the characteristics. Reduction to a single statistic, however ingeniously constructed, is equivalent to replacing a multivariate problem by a univariate one, with attendant loss of information. The possibility of hiding important information is much greater when multivariate characteristics are under consideration, and the desirability of using vector valued PCIs arises quite naturally.
