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.