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EXPONENTIALLY
WEIGHTED MOVING AVERAGE (EWMA) CHARTS
An EWMA (Exponentially Weighted
Moving-Average) Chart is a control chart for variables data (data that is
both quantitative and continuous in measurement, such as a measured dimension
or time). It plots weighted moving average values. A weighting factor is
chosen by the user to determine how older data points affect the mean value
compared to more recent ones. Because the EWMA Chart uses information from
all samples, it detects much smaller process shifts than a normal control
chart would.
Although standard EWMA charts
are designed to monitor processes with a stable mean, a modified EWMA control
charts may be used for autocorrelated processes with a slowly drifting mean.
See also:
SPC Concepts
When
to Use an EWMA Chart
Interpreting
an EWMA Chart
For a general discussion
of EWMA charts, refer to any Statistical Quality Control textbook, such
as (Montgomery, 1991).
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