A comparative study between the variable sampling interval X and Double Sampling X Charts

The Shewhart X̅ control chart is a useful process monitoring tool in manufacturing industries to detect the presence of assignable causes. However, it is insensitive in detecting small process shifts. To circumvent this problem, adaptive control charts are suggested. An adaptive chart enables at least one of the chart's parameters to be adjusted to increase the chart's sensitivity. Two common adaptive charts that exist in the literature are the double sampling (DS) X̅ and variable sampling interval (VSI) X̅ charts. This paper compares the performances of the DS X̅ and VSI X̅ charts, based on the average time to signal (ATS) criterion. The ATS profiles of the DS X̅ and VSI X̅ charts are obtained using the Mathematica and Statistical Analysis System (SAS) programs, respectively. The results show that the VSI X̅ chart is generally superior to the DS X̅ chart.