A Variable Sampling Interval Shewhart Control Chart for Monitoring the Coefficient of Variation in Short Production Runs

Monitoring the coefficient of variation (CV) allows process monitoring to be performed when both theprocess mean and the standard-deviation are not constant but, nevertheless, proportional. Until now,few research papers have investigated the monitoring of the CV in a short production run context. This paper investigates the design and implementation of a Variable Sampling Interval (VSI) Shewhart control chart to monitor the coefficient of variation in a short production run context. Formulas for the truncated average time to signal are derived and a performance comparison is carried out with a Fixed Sampling Rate Shewhart chart monitoring the CV. An example illustrates the use of this chart on real industrial data.