A variable sampling interval S2-EWMA control chart for monitoring the process variance
This paper proposes a Variable Sampling Interval version of the Fixed Sampling Interval S2-EWMA control chart developed by Castagliola (2004) and dedicated to the monitoring of the sample variance of a process. In this paper, we explain how the various parameters of this VSI S2-EWMA control chart can be computed and how the use of the VSI feature significantly improves the statistical efficiency of FSI S2-EWMA chart, thus representing an effective tool in the detection of process out-of-control conditions. An optimal design strategy based on the Average Time to Signal (ATS) is presented and a comparison with the FSI procedure is performed.
Keywords: sample variance, exponentially weighted moving average, EWMA control charts, logarithmic transformation, adaptive sampling interval, variable sampling interval, process monitoring, SPC, statistical process control, quality control, optimal design