Monitoring simultaneously two or more indices of health care. Multivariate quality control procedures

Often the data collected in a health care evaluation program consist of a number of indices, each of which represents a different component of a health care process. The question, "Is the health care process in control," to be answered effectively, must be answered in terms of the effect of the several indices considered jointly rather than in terms of each variable considered separately. This article describes the nature of the problem of monitoring jointly several indices of health care and presents two quality control methods, the control ellipse and Hotelling's T2, which are applicable to the multivariable setting. The discussion focuses on the case of monitoring simultaneously two related indices of health care, and extension of the two techniques to more than two indices is discussed briefly.