The center line represents the average standard deviation for a subgroup.
If you do not use an unbiasing constant, then Minitab uses the process standard deviation, σ:
If you use an unbiasing constant(default), then Minitab calculates the center line value for each subgroup, i, as follows:
|σ||process standard deviation|
|c4()||value of the unbiasing constant c4 that corresponds to the value that is specified in parentheses.|
|ni||number of observations in subgroup i|