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Moving
Range Chart Calculations
Plotted statistic
The
moving ranges between successive subgroups in an Individual-X Chart (i.e:
the difference between the current observation and the observation immediately
prior).
Center Line
where m is the total number
of subgroups included in the analysis and MRj is the Moving Range at subgroup
j.
Note: When control limits
for the Individual-X chart are defined as fixed values (such as when historical
data is used to define control limits), the Average Moving Range (MR-bar)
must be back calculated from these pre-defined control limits. This ensures
that the control limits on the Moving Range chart are at the same sensitivity
as those on the Individual-X chart. In this case:
where d2 (available in
any statistical quality control textbook) is based on n=2.
UCL
, LCL (Upper and Lower Control Limit)
(All Distributions)
(All Distributions)
where MR-bar is the Average
of the Moving Ranges , sigma-x is the process
sigma , and d3 is a function of n (available in any statistical quality
control textbook).
Notes: Some authors prefer
to write this as:
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