X Chart - Options - Estimate
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Stat > Control Charts > Variables Charts for Subgroups > Xbar Chart  > Xbar Options > Estimate

Allows you to omit or use certain subgroups to estimate m and s. 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. You can also select one of two methods to estimate s, force the control limits to be constant when you have unequal sample sizes, and use biased estimates of s.

When you include or exclude rows using control chart Options > Estimate and choose to perform a Box-Cox transformation, Minitab only uses the non-omitted data to find lambda.

You can set preferences for the estimation of s using Tools > Options > Control Charts and Quality Tools > Estimating Standard Deviation.

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 entire data set, not just 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 Minitab considers an omitted sample (subgroup 1) as part of the entire data set, you must enter subgroup 2 as the subgroup to omit.

Dialog box items

Omit the following subgroups when estimating parameters (eg, 3 12:15): Choose to specify subgroups of data to omit from the computations for m and s, then enter individual subgroup numbers or a range of subgroups. Enter entire subgroups, not individual observations. To omit one observation from a subgroup, change that value to missing (*).

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

Method for estimating standard deviation

Subgroup size > 1

Rbar: Choose to estimate s based on the average of the subgroup ranges.

Sbar: Choose to estimate s based on the average of the subgroup standard deviations.

Pooled standard deviation: Choose to estimate s using a pooled standard deviation.

Use unbiasing constants: Check to use unbiasing constants; uncheck to use biased estimates of s.