Phase II Quality Control Charts for Detecting Small Shifts

This chapter examines phase II control charts, which are used in a matured process and when the statistical process control implementation of phase I is over. Cumulative sum (CUSUM) and exponentially weighted moving average (EWMA) control charts can be used to monitor the process mean as well as the process variance, fraction nonconforming, and nonconformities. CUSUM control charts can be implemented by using either a numerical procedure or a graphical procedure. No process is immune to changes in variability, and this is important to note when CUSUM control charts are implemented. A EWMA control chart is very useful for processes when only one observation per period is available. The EWMA control chart uses information from all observations made up to and including time i, as is done with CUSUM control charts.