Hotelling’s T2 control chart with double warning lines

Recent studies have shown that the T ² control chart with variable sampling intervals (VSI) and/or variable sample sizes (VSS) detects process shifts faster than the traditional T ² chart. This article extends these studies for processes that are monitored with VSI and VSS using double warning lines (T ² -DWL). It is assumed that the length of time the process remains in control has exponential distribution. The properties of T ² -DWL chart are obtained using Markov chains. The results show that the T ² -DWL chart is quicker than VSI and/or VSS charts in detecting almost all shifts in the process mean.