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 (F_{1 }– 1) |
X = X – i (F_{1 }– 1) |
X = M – i (F_{1 }– 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.

F^{1 }= Average of the cumulative frequencies, *i.e.* ∑GF/N