Combined double sampling and variable sampling interval (X)over-bar chart

In recent years several studies have shown that the (X) over bar chart with variable sampling intervals (VSI), the (X) over bar chart with variable size (VSS), the (X) over bar chart with variable sample size and sampling intervals (VSSI) and the (X) over bar chart with variable parameters (VP) detect both small and moderate shifts in the process mean more quickly than the traditional Shewhart (X) over bar chart. Double sampling is the counterpart to double sampling plans. A combined double sampling variable sampling interval (DSVSI) (X) over bar chart is studied in this paper. It is compared with the aforementioned charts and with the CUSUM and EWMA charts. In all cases, the DSVSI (X) over bar chart is quicker at detecting small and moderate shifts in the process mean. An example is provided.