Under this method, computations are made purely on the basis of the frequency column and the common interval of the classes. This method is advantageously used only where the values of a variable are stated in equal intervals, and are arranged in ascending order. If a series stands in a descending or disorderly manner, it is to be rearranged first in the ascending order. Further, in case of individual series, a frequency column is to be introduced by assigning a unit frequency i.e. against each item of the series. The formula to be applied under this method are as follows:
|Individual Series||Discrete Series||Continuous Series|
|X = X – i (F1 – 1)||X = X – i (F1 – 1)||X = M – i (F1 – 1)|
Where, X = Last value of the variable arranged in ascending order
M = Mid value of the last class arranged in ascending order
i = Interval of equal magnitude.
F1 = Average of the cumulative frequencies, i.e. ∑GF/N