X = the mean of the individual measurements

MR = the mean of the nonmissing moving ranges computed as (MR2+MR3+...+MRN)/(N-1) where MRi = |xi - xi-1|.

σ = the process standard deviation

d2(n) = expected value of the range of n independent normally distributed variables with unit standard deviation.

d3(n) = standard deviation of the range of n independent observations from a normal population with unit standard deviation.