Cusum and EWMA Charts for Detecting Small Shifts in Process

Jay Arthur
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Shewart control charts may not be sensitive enough to detect small changes in the process mean. When greater sensitivity is required, consider using the Cusum or EWMA chart.

Cumulative Sum (Cusum Chart)

A Cusum chart plots the cumulative sum of deviations of successive sample means from a target. When running a Cusum Chart in the QI Macros, the macros will prompt you for a target value. The Cusum analyzes variation from that target value looking for cumulative shifts away from the center line.

Here is an example of the same data plotted using an XmR and a Cusum chart. Notice how the Cusum chart more quickly and clearly identifies the process shift.

Exponetially Weighted Moving Average (EWMA) Chart

The EWMA Chart plots the moving averages of data and assigns weights that decrease exponentially from the present to the past. As a result, the average values are influenced more by recent data points than older points. When running an EWMA chart in the QI Macros, the macros will prompt you for a weight between 0 and 1. The default value is 0.2.

Here is an example of an EWMA chart and XmR chart plotted using the same data.

Both the Cusum Chart and EWMA Chart can be found on the QI Macros pull down menu under control charts.

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