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.
