A Nonparametric Control Chart for Process Variability Based on Quantiles
Most of the control charts are based on assumption of normality. Control charts for non-normal process distributions have also been reported in literature. In absence of any knowledge about the process distribution, nonparametric chart is a good alternative. In the recent past number of nonparametric control charts have been studied. In the present work we propose a control chart for monitoring process variability, which is based on in-control quantiles. The chart is motivated from a nonparametric control chart based on in-control quartiles due to Amin et al. (1995). The proposed chart has been studied for its performance for various process distributions to monitor change in variability and has been compared with the existing nonparametric and parametric charts. It has attractive out-of-control Average Run Length performance and is very simple to use. We illustrate the chart through an example and recommend use of this chart to monitor process variability. Generalization of the chart will also be discussed in view to further improve its detection ability.
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