Can control charts detect small process shifts?
Typical control charts, such as Xbar & R and X & Mr, indicate whether or not a process is statistically stable. They detect process changes greater than 1.5σ and, because of this, are not the best tool to detect small process shifts. To detect small shifts, defined as shifts less than 1σ, use the EWMA or Cusum charts. The EWMA (Exponential Weighted Moving Average) chart weights moving average values according to time. The more recent the value, the greater the weight assigned to that value and, in effect, the greater the emphasis given to that value. The Cusum (Cumulative Sum) chart plots the cumulative sum of deviations from a target value. These charts detect shifts from the process mean as small as 0.5σ. Cusum charts are plotted in 2 forms – V-Mask and Tabular Cusum (available in WinSPC). Although EWMA and Cusum charts are very valuable in monitoring continuous processes where small changes have significant effects, it is not recommended that they be used as process improvement tool
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