A New Chart for Detecting the Process Mean and Variability

Since the earliest days of statistical process control, it has been recognized that it is essential to monitor both mean and variability and to trigger an alarm if either characteristic shows sign of a special cause. It is, however, possible to monitor both mean and variance in a single chart responsive to shifts in either. Traditionally, this has been done by using separate X̄ and S or R charts. The purpose of this paper is to propose a new single chart with the Shiryaev-Roberts (SR) procedure to monitor both mean and variance. It is shown that, in addition to the simplicity of a single chart rather than two, the new chart has some advantages over many other charts in detecting process changes.