Question

Need an explanation on how the D value is calculated. See attached example. Why is the range 16.9-22.5 the D value of 0?

* CF is cumulative frequency (82 10+ 21 +51) Midpoint Classes 141 413.7 11.3 16.9 16.9 - 22.S 22.5 - 28.1 28.1 33.7 33.7-39.3 39.3 -44.9 Total 10 31 82 90 115 130 130458 14.1 19.7 25.3 1,290.31 30.9 36.5 42.1 168.6 3,636.29 10 21 51 247.2 912.5 631.5 2 25 15 4

Answer 1

For simplicity in computation, a transformation is often taken from class marks(x) to D such that x*f(x) Or x^2*f(x) figures become smaller in magnitude. For that purpose a value is subtracted from x and then divided by the class width ( in case of equal class widths)  

Here, D=(x - 19.7) /5.6, because 5.6 is the common class width of each class.