Interpretation & Calculations
The exponentially weighted moving average of the current subgroup, as used in a standard EWMA Chart.
λ is the value of the Weighting Factor (user defined; See When to Use EWMA Charts )
x-bart is the subgroup average, for the current subgroup at time t (or the current observation, if subgroup size is one (n=1)).
the value of z at time zero (zo) is either a Target value or the overall Average of the selected subgroups (user defined).
Learn more about the SPC principles and tools for process improvement in Statistical Process Control Demystified (2011, McGraw-Hill) by Paul Keller, in his online SPC Concepts short course (only $39), or his online SPC certification course ($350) or online Green Belt certification course ($499).