LCL for X chart =

UCL for X chart =

LCL for S-chart =

UCL for S-chart =

Center line for S-chart: By default, the center line for the ith subgroup (where 3 is the sigma multiplier) indicates an estimate of the expected value of si. This value is computed as , where is an estimate of σ.

σ = process standard deviation

c4(n) is the expected value of the standard deviation of n independent normally distributed variables with unit standard deviation

c5(n) is the standard deviation of the standard deviation of n independent observations from a normal population with unit standard deviation

N is the number of subgroups for which