Hotelling's T² control chart with variable sampling intervals

Hotelling's T² chart is the most widely used multivariate procedure to control changes in the mean vector of p correlated quality characteristics. In this paper, it is shown how to increase the efficiency of the test using the variable sampling interval technique. Given a control scheme with constant size and frequency of sampling, tables are produced that give a scheme, with two possible periods of time between samples, that is equivalent to the standard procedure when the process is in control. However, the new chart, as it uses two possible intervals between samples, is faster in detecting a change in the process. Various alternatives are given for the possible users of this method.