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EWMA Control Charts

EWMA Control Charts are based on an Exponentially Weighted Moving Average. The control limits are:

 Steady State Upper Control Limit Lower Control Limit Initial Readings Upper Control Limit Lower Control Limit

L is the number of standard deviations to the control limits

i is the number of periods

λ is  a smoothing constant 0<λ<1

The 'Steady State' control limits are normally sufficient, however ten or so points are required before the smoothed average stabilizes on these values. The 'Initial Readings' limits should be used for the first few points plotted after a process change.

 The selection, design and use of control charts is covered in the MiC Quality courses Statistical Process Control and Advanced Statistical Process Control. Try out our courses by taking the first module of the Primer in Statistics free of charge.
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