Minitab provides four methods for estimating the process standard deviations. You should choose an estimation method based on the properties of your particular process/product. You can also choose to enter a historical value in the Parameters dialog box.
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.