Indirect method of mean
Also called: Assumed mean method · Topic: Geographical Data: Sources, Compilation and Processing · NCERT: Class 12, Ch 2 "Data Processing"
Meaning
The indirect method of mean is a shortcut for finding the mean when the numbers are large. You pick a convenient value, called the assumed mean (A). You subtract A from each value to get small deviations (d); this step is called coding. Then mean = A + Σd ÷ N, where Σd is the sum of the deviations and N is the number of values. It gives the same answer as the direct method, with easier sums, and works for grouped data with equal or unequal class intervals.
Example
The normal rainfall of seven districts of the Malwa plateau in Madhya Pradesh ranges from 825 mm (Mandsaur) to 1,083 mm (Dewas). Taking A = 800 mm, the deviations add up to 884, so the mean = 800 + 884 ÷ 7 = 926.29 mm.
Don't confuse with
- Direct method of mean: adds all the actual values and divides by N (6,484 ÷ 7 = 926.29 mm), with no assumed mean.