Hotelling’s T2 control chart with double warning lines

Recent studies have shown that the T2 control chart with variable sampling intervals (VSI) and/or variable sample sizes (VSS) detects process shifts faster than the traditional T2 chart. This article extends these studies for processes that are monitored with VSI and VSS using double warning lines (T2—DWL). It is assumed that the length of time the process remains in control has exponential distribution. The properties of T2—DWL chart are obtained using Markov chains. The results show that the T2—DWL chart is quicker than VSI and/or VSS charts in detecting almost all shifts in the process mean.