On the properties of the Hotelling’s T2 control chart with variable sampling intervals

When T² control chart is used to monitor a process, it is usually assumed that the samples of size n₀ is taken at constant intervals t₀. In this paper, we investigate the T² control chart for monitoring the process mean vector when the sampling intervals are variable. Recent studies have shown that the variable sampling interval (VSI) scheme helps practitioners detect process shifts more quickly than the classical scheme Fixed Ratio Sampling (FRS). In this paper, it is assumed that the length of time the process remains in control is exponentially distributed.