This function returns a factor (Quality Control D2 Factor)
It estimates the standard deviation of a parent distribution (or population) from an average range when the sample size (or subgroup size) n is given.
The standard deviation of the parent distribution = average range/factor.
The returned factors assume sampling from a normal population.
double QCD2(int n)
Returns Quality Control D2 Factor.
QCD2(4) =; // returns 2.05875, the Quality Control D2 factor for a sample size of 4