Adaptive R charts with variable parameters

The Shewhart R control chart (R chart) has been widely used to monitor process variance. However, the main disadvantage of an R chart is its slowness to signal small increases on the variability. In this paper, ideas of adaptive control charts are extended to the Shewhart R chart for improving efficiency in signaling increases on the variance. A Markov chain model is applied to evaluate its performance. The statistical performance shows that the R chart with variable parameters (VP) is more sensitive to variance increases.