A variable sample size run sum coefficient of variation chart

AbstractA run sum chart with variable sample size (VSS) to detect shifts in the coefficient of variation (γ) is proposed. The run sum γ chart divides the chart into different regions with distinct scores assigned to these regions, and evaluates the process from the cumulative score. The proposed chart varies the sample size based on the cumulative score. A detailed description of the operation for the proposed chart is provided. Formulae to evaluate various performance measures are also derived. Furthermore, algorithms to optimize the chart are proposed. The chart significantly outperforms the run sum γ chart without the VSS feature in most cases. Besides the run sum γ chart, comparisons are also made against other γ charts. Finally, the implementation of the VSS run sum γ chart is shown on an illustrative example.