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Specify observations to estimate parameters
If some observations have erratic data because of special causes that you have corrected, you can omit these observations from the calculations to avoid incorrect parameter estimates.
- From the drop-down list, specify whether you want to list the observations to omit, or the observations to include.
- To list the observations to omit, select Omit the following subgroups when estimating parameters (eg, 3 12:15).
- To list the observations to include, select Use the following subgroups when estimating parameters (eg, 3 12:15).
- List the observations. Use a colon to indicate a range of observations. Leave a space between each observation or range of observations. For example, to specify observations 2, 9, 44, 45, 46, and 47, enter 2 9 44:47.
Method for estimating standard deviation
Choose the method to estimate the standard deviation that Minitab uses to calculate the control limits.
- Subgroup size = 1
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- Average moving range: The average moving range is the average value of the moving range of two or more consecutive points. Use this method when the subgroup size is 1.
- Median moving range: The median moving range is the median value of the moving range of two or more consecutive points. Use this method when the data have extreme ranges that influence average of the moving ranges.
- Square root of MSSD: The square root of MSSD is the square root of the mean of the squared differences between consecutive points. Use this method when you cannot reasonably assume that at least 2 consecutive points were collected under similar conditions.
Specify length of moving range
In Length of moving range, enter the number of observations that are used to calculate the moving range. By default, a length of 2 is used because consecutive values have the greatest chance of being alike. The length must be 
100.
If the data are cyclical, consider using a moving range of a different length. For example, if you collect quarterly data, consider using a moving range of length 4 to ensure that one observation from each season is included in the calculation.
Use Nelson estimate
Use unbiasing constant
Select Use unbiasing constant to eliminate bias that can occur when you estimate certain process parameters. Often, the choice to use unbiasing constants depends on company policy or industry standards.
