A Variable Sampling Interval Side-sensitive Synthetic Coefficient of Variation Chart

The side-sensitive synthetic chart has been shown to effectively detect changes in the coefficient of variation , though its reliance on fixed sampling intervals may reduce its efficiency when compared to adaptive strategies. This paper aims to improve the chart by incorporating the variable sampling interval (VSI) feature that varies the sampling interval based on the performance of the plotting statistic of the previous sample. Formulae for the average and standard deviation of the time to signal (ATS and SDTS) of the proposed chart are derived, and procedures that optimise the chart’s performance are given. Numerical comparisons reveal that the VSI feature enhances the performance of the side-sensitive synthetic 𝛾 chart by resulting in smaller out-of-control ATS values. Furthermore, an actual example is given to illustrate the effectiveness of the proposed chart in detecting out-of-control conditions in practical settings.