Monitoring the Coefficient of Variation Using Control Charts with Run Rules

Monitoring the coefficient of variation (CV) is a successful approach to Statistical Process Control when the process mean and standard deviation are not constant. In recent years the CV has been investigated by many researchers as the monitored statistic for several control charts. Viewed under this perspective, this paper presents a new efficient method to monitor the CV by means of Run Rules (RR) type charts. Tables are provided to show the statistical run length properties of Shewhart- gamma, RR2,3-gamma, RR3,4-gamma and RR4,5-gamma control charts for several combinations of in control CV values gamma(0), sample size n and shift size tau. Indeed, comparative studies have been performed to find the best control chart for each combination. An example illustrates the use of these charts on real data gathered from a metal sintering process.