EWMA Control Chart

Exponentially Weighted Moving Average

See Also:

Description:

The exponentially weighted moving average (EWMA) control chart is an alternative to the Shewhart-CUSUM control chart. It was first introduced in the 1950for use in statistical quality control applications. This implementation is based on information provided by Montgomery (1996).

Use:

The EWMA provides a graphical method that is not sensitive to normality requirements, and therefore can be used with non-normal data distributions, or moderate percentages of non-detects. As implemented in ChemStat, the EWMA Control Chart may be used for either intra-well or inter-well comparisons.

Options:

Select MA Control Chart Options from the Options menu or from the right-mouse-button context menu to enter values of L, lambda, and group size(n). Also entered from this option is the number of baseline samples. If a positive number of baseline samples is entered, the baseline samples are counted from the first sampling date. If a negative number is entered, the number of baseline samples is equal to the total number of samples for the well minus the absolute value of the number entered. Baseline samples are not graphed, but are used only to establish baseline mean and standard deviation.

There is also an option to circle points that exceed the control limits. This can make the chart easier to interpret by persons unfamiliar with control charts.

Remarks:

If the group size (n) is 1, each individual concentration is compared to the control limit. A larger group size will average consecutive concentrations for the comparison, and help to control for outlier concentrations.

Like the Shewhart-CUSUM control chart, the EWMA control chart reacts quickly to small changes, but is not as sensitive to large changes as the Shewhart chart.