Single Variables Control Charts: A Further Overview

Control charts are widely used in industries to monitor a process for quality improvement. When dealing with variables data, we usually employ two control charts to monitor the process location and spread since one of them or both may change. Furthermore, a change in the process variability can affect effectiveness of the control chart for the mean as its control limits will change. We give an overview of control charts that are proposed in the last decade or so in an effort to use only one chart to simultaneously monitor both process location and spread. Two approaches have been advocated for using one control chart for process monitoring. One approach plots two quality characteristics in the same chart while the other uses one plotting variable to represent the process location and spread.