Options - Estimate
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... > control chart Options > Estimate

Charts  Moving Range, I-MR

Omits or includes certain observations to estimate m and s. For example, if some observations are erratic due to assignable causes that you have corrected, you can prevent them from influencing the estimation of m and s. You can also select one of two methods to estimate s, change the length of the moving range, 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 and the length of the moving range 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 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: Choose to specify observations to omit from the computations for m and s, then enter individual observation numbers.

Use the following subgroups when estimating parameters: Choose to specify observations to compute m and s, then enter individual observation numbers.

Method for estimating standard deviation

Subgroup size = 1

Average moving range: Choose to estimate s using the average of the moving range.

Median moving range: Choose to estimate s using the median of the moving range.

Length of moving range: Enter the number of observations to use to calculate the moving range. By default, Minitab uses a span of two because consecutive values have the greatest chance of being alike. The span must be < 100.

Use Nelson estimate: (This option is only available with I Chart and I-MR Chart.) Check to correct for unusually large moving range values in the calculation of the control limits. The procedure is similar to the procedure proposed by Nelson [23]. Minitab eliminates any moving range values that are more than 3s larger than the average moving range, then recalculates the average moving range and control limits. Minitab repeats this procedure until no moving range values are more than 3s larger than the average moving range.