Monitoring the Process Mean and Variance Using Individual Observations and Variable Sampling Intervals

Summary: [This abstract is based on the authors' abstract. A longer abstract is available on the journal's website.]

Control charts are evaluated for monitoring a process to detect changes in the mean and/or variance of a normal quality variable when a separate observation is taken at each sampling point. A combination of the traditional X chart and moving range (MR) chart did not detect small and moderate parameter shifts as fast as combinations involving the exponentially weighted moving average (EWMA) chart. In addition, EWMA combination charts were as effective as X and MR combination charts in diagnosing parameter shifts produced by a special cause. The addition of the variable sampling interval (VSI) feature to the combinations of charts resulted in substantial reductions in the time required to detect shifts in process parameters.