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MRCHART Statement |
A subgroup range Ri is included in the calculation
only if .The unbiasing factor d2(ni) is defined so that,
if the observations are
normally distributed, the expected value
of Ri is equal to
.Thus,
is the unweighted average of N unbiased
estimates of
.This method is described in the ASTM Manual on Presentation of
Data and Control Chart Analysis (1976).
A subgroup range Ri is included in the calculation only
if , and N is the number of subgroups for which
ni geq 2.
The MVLUE assigns greater weight to estimates of
from
subgroups with larger sample sizes, and it is intended for
situations where the subgroup sample sizes vary. If the subgroup
sample sizes are constant, the MVLUE reduces to the default
estimate.
See Example 36.1 for illustrations of the default and MVLUE methods.
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