EWMA Control Limits

The upper & lower control limits for the EWMA statistic :

where zo is the Starting value (user defined as either the Target value or process mean value), n is the subgroup size, d2 is a function of n, and m is the number of subgroups selected for analysis.

The Range chart (n>1) or the Moving Range chart (n=1) is generally used to monitor the process variation.

See also:

Interpreting an EWMA Chart

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).