Variable sampling interval Hotelling's T-2 chart with sampling at fixed times

This study applies a sampling at fixed time feature to the variable sampling interval (VSI) Hotelling's T-2 chart for rational subgroups. This control chart is called variable sampling interval with sampling at fixed times (VSIFT) control chart, in which the modification allows one to take additional samples between two fixed equally spaced time points when there is some indication of process mean shift for the multivariate process. From a practical viewpoint, the VSIFT control chart is easier to administer than the adaptive sampling interval control charts, i.e. VSI, variable sample size and sampling interval (VSSI), and variable parameters control charts. Numerical comparisons show that the VSIFT Hotelling's T-2 chart performs better than the VSI, variable sample size, VSSI, variable parameters, and variable sample size and control limit Hotelling's T-2 charts for detecting moderate and large shifts in the process mean in terms of the steady-state average time to signal.