AN APPLICATION OF VARIABLE SAMPLING INTERVAL CONTROL CHARTS

Variable Sampling Interval (VSI) control charts have been studied extensively, and It has been shown for Shewhart, cumulative sum (CUSUM), and exponentially weighted moving average (EWMA) charts that shifts in the process mean can be detected more quickly with no increase in the average in-control sampling or false alarm frequency relative to fixed interval charts. Applications of VSI charts in industry have been few, however, primarily due to logistical problems associated with a variable sample schedule. In this paper, we present an application of a VSI-EWMA chart at Monsanto's nylon fiber plant in Pensacola, Florida. Motivation for this implementation, instead of being quicker detection of shifts, was a significant decrease in laboratory cost with little adverse impact on control chart performance compared with the fixed-interval EWMA chart. The logistical problems were solved by employing a fixed time option with the VSI scheme (VSI-FT), where there is a fixed sample schedule for the long-interval, and in-between samples are taken when warranted by the data.