Monitoring the Coefficient of Variation Using a Variable Sample Size Control Chart in Short Production Runs

Monitoring the coefficient of variation (CV) is an effective approach to monitor aprocess when both the process mean and the standard deviation are not constant but,nevertheless, proportional. Until now, few contributions have investigated themonitoring of the CV for short production runs. This paper proposes an adaptiveShewhart control chart implementing a Variable Sample Size strategy (VSS) in order tomonitor the coefficient of variation in a short production run context. Formulas for thetruncated average run length are derived. Moreover, a comparison is performed with aFixed Sampling Rate Shewhart chart for the CV in order to evaluate the performance ofeach chart in a short run context. An example illustrates the use of this chart on realdata.