Monitoring the Coefficient of Variation using a Variable Sample Size Control Chart

This paper proposes an adaptive Shewhart control chart implementing a variablesample size strategy in order to monitor the coefficient of variation. The goals of thispaper are: a) to propose an easy-to-use 3-parameters logarithmic transformation forthe coefficient of variation in order to handle the variable sample size aspect, b) toderive the formulas for computing the average run length, the standard deviation runlength and the average sample size and to evaluate the performance of the proposedchart based on these criteria, c) to present ready-to-use tables with optimal chartparameters minimizing the out-of-control average run length as well as the out-ofcontrolaverage sample size, d) to compare this chart with the fixed sampling rate,variable sampling interval and synthetic control charts. An example illustrates the useof the variable sample size control chart on real data gathered from a casting process.