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

Charts  Zone, Moving Average, EWMA, CUSUM

Omit or includes 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 of process parameters. You can also select one of several 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 also set preferences for the estimation of s and the length of the moving range for all Minitab sessions using Tools > Options > Control Charts and Quality Tools > Estimating Standard Deviation. When you set options, all affected dialog box settings automatically reflect your preferences.

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 subgroups of the data to omit from the computations for m and s, then enter individual subgroup numbers or ranges of subgroups. Enter entire subgroups, not individual observations. To omit one observation from a sample, change that value to missing (*) in the worksheet.

Use the following subgroups when estimating parameters: 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

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.

Square root of MSSD:  Choose to estimate s using the square root of MSSD (mean of the squared successive differences).

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.

Length of moving range: Check to enter the number of observations used to calculate the moving range. By default, a span of two is used because consecutive values have the greatest chance of being alike. The span must be < 100.

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