D Chart: An Efficient Alternative to Monitor Process Dispersion

Control chart is the most important Statistical Process Control tool used to monitor reliability and performance of industrial processes. For monitoring changes in process dispersion, the R and S charts are widely used. These control charts perform better under the ideal assumption of normality but are well known to be very inefficient in presence of outliers or departures from normality. In this study we propose a new control chart for monitoring process dispersion, namely the D chart, and compared its performance with the R and S charts using probability to signal as a performance measure. It has been observed that the newly proposed chart is superior to the R chart and is a close competitor to the S chart under normality of quality characteristic. When the assumption of normality is violated, the D chart is more powerful than both the R and S charts in terms of detecting shifts in process dispersion. This study will help quality practitioners to choose an efficient alternative to the classical R and S charts for monitoring dispersion of industrial processes.