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When
to Use a Moving Average & Range Chart
Moving Average Charts are
generally used for detecting small shifts in the process mean. They will
detect shifts of .5 sigma to 2 sigma much faster than Shewhart charts with
the same sample size. They are, however, slower in detecting large shifts
in the process mean. In addition, typical run tests cannot be used because
of the dependence of data points.
Moving Average Charts
may also be preferred when the subgroups are of size n=1. In this case,
an alternative chart might be the Individual X Chart, in which case you
would need to estimate the distribution of the process in order to define
its expected boundaries with control limits. The advantage of Cusum, EWMA
and Moving Average charts is that each plotted point includes several observations,
so you can use the central limit theorem to say that the average of the
points (or the moving average, in this case) is normally distributed and
the control limits are clearly defined.
Another use of the Moving
Average Charts is for processes with known intrinsic cycles. Many accounting
processes and chemical processes fit into this categorization. If you sample
at set intervals and set the cell size equal to the number of subgroups
per cycle, then as you drop the oldest sample in the cell, you pick up the
corresponding point in the next cycle. If the cyclical nature of the process
is upset, then the new points added will be substantially different, causing
out of control points.
As with other control
charts, Moving Average Charts are used to monitor processes over time. The
charts' x-axes are time based, so that the charts show a history of the
process. For this reason, you must have data that is time-ordered; that
is, entered in the sequence from which it was generated. If this is not
the case, then trends or shifts in the process may not be detected, but
instead attributed to random (common cause) variation.
Moving Average & Range
Charts may be used when the cell size is less than ten subgroups. If the
cell size is greater than ten, use Moving Average & Sigma charts. Other
charts useful in the above scenarios are the EWMA and Cusum charts.
See also:
Autocorrelation
Charts
EWMA
Charts
CuSum
Charts
Moving
Average / Range Chart
Moving
Average / Sigma Chart
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