An Improved R (Range) Control Chart for Monitoring the Process Variance

An R chart is often used to monitor shifts in the process variability. However, the range, , statistics from a sampling distribution are highly skewed. Hence, the classical R chart based on the control limits will not give an in‐control average run length of approximately 370, or equivalently a type I error, . In this paper, an approach is shown to obtain the control limits of an improved R chart based on a desired type I error from the density function of the R_i statistics. Copyright © 2004 John Wiley & Sons, Ltd.