A Variable Sampling Interval Synthetic Xbar Chart for the Process Mean

The usual practice of using a control chart to monitor a process is to take samples from the process with fixed sampling interval (FSI). In this paper, a synthetic [Image: see text] control chart with the variable sampling interval (VSI) feature is proposed for monitoring changes in the process mean. The VSI synthetic [Image: see text] chart integrates the VSI [Image: see text] chart and the VSI conforming run length (CRL) chart. The proposed VSI synthetic [Image: see text] chart is evaluated using the average time to signal (ATS) criterion. The optimal charting parameters of the proposed chart are obtained by minimizing the out-of-control ATS for a desired shift. Comparisons between the VSI synthetic [Image: see text] chart and the existing [Image: see text] , synthetic [Image: see text] , VSI [Image: see text] and EWMA [Image: see text] charts, in terms of ATS, are made. The ATS results show that the VSI synthetic [Image: see text] chart outperforms the other [Image: see text] type charts for detecting moderate and large shifts. An illustrative example is also presented to explain the application of the VSI synthetic [Image: see text] chart.