U Chart Options - Estimate
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...  > control chart Options > Estimate

Charts  U, Laney U'

You can omit certain subgroups from the estimation of u. For example, if some subgroups have erratic data due to assignable causes that you have corrected, you can prevent these subgroups from influencing the estimation of process parameters. You can also force the control limits to be straight if you have unequal sample sizes.

Caution

When you include or exclude rows using control chart > Data Options > Subset and also omit or use samples to estimate parameters, the omitted or used samples apply to the subsetted data. For example, you exclude subgroup 1 from your analysis (using control chart > Data Options > Subset), then decide to omit subgroup 2 from the parameter estimation (using control chart Options > Estimate). Because subgroup 1 is excluded from the control chart, the original subgroup 2 becomes the new subgroup 1. When you use control chart Options > Estimate, enter subgroup 1 as the subgroup to omit from parameter estimation.

Dialog box items

Omit the following subgroups when estimating parameters (eg, 3 12:15): Choose to specify subgroups of the data to omit from the estimation of u, then type individual subgroup numbers or ranges of subgroups.

Use the following subgroups when estimating parameters (eg, 3 12:15): Choose to specify subgroups of data to compute u, then type individual subgroup numbers or a range of subgroups.