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SCHART Statement |
A subgroup standard deviation s_{i} 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 n_{i} - 1. A subgroup standard deviation s_{i} 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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