This function returns a factor. (Quality Control D4 Factor)
It determines the 3-sigma upper control limit for R charts (Range of Sample Charts) from the average range when the sample size (or subgroup size) n is given.
The Upper Control Limit for R = (factor)*(Average Range).
The calculations for the factors are based on the normal distribution.
double QCD4(int n)
Returns Quality Control D4 Factor
QCD4(11) =; //returns 1.744, the Quality Control D4 factor for a sample size of 11