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XSCHART Statement |
A subgroup standard deviation si
is included in the calculation only
if , and N is the number of subgroups
for which
.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.
The weights
are the degrees of freedom ni - 1.
A subgroup standard deviation si is included in the
calculation only if , and N is the number of
subgroups for which
.
If the unknown standard deviation is constant across
subgroups, the root-mean-square estimate is more efficient
than the minimum variance linear unbiased estimate. However,
in process control applications it is generally not assumed that
is constant, and if
varies across subgroups, the root-mean-square estimate tends
to be more inflated than the MVLUE.
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