If some subgroups have erratic data because of special causes that you have already corrected, you can omit these subgroups from the calculations to avoid incorrect parameter estimates.
Choose the method to estimate the standard deviation that Minitab uses to calculate the control limits.
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.