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.
