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20 Standard SPC Control Charts
EWMA Chart – Exponentially Weighted Moving Average
The EWMA chart is an alternative to the preceding Shewhart type control charts (X-Bar
R and I-R charts in particular) and is most useful for detecting small shifts in the process
mean. It uses a weighted moving average of previous values to “smooth” the incoming
data, minimizing the affect of random noise on the process. It weights the current and
most recent values more heavily than older values, allowing the control line to react
faster than a simple MA (Moving Average) plot to changes in the process. Like the
Shewhart charts, if the EWMA value exceeds the calculated control limits, the process is
considered out of control. While it is usually used where the process uses 100%
inspection and the sample subgroup size is 1 (same is the I-R chart), it can also be used
when sample subgroup sizes are greater than one.
MA (Moving Average) Chart with Sample Values Plotted