Apportionment Method of Population Forecasting (Ratio Method)

Ratio method of population forecasting, also known as apportionment method is used for cities showing same development curve as that of the nation.

Ratio or Apportionment Method

Assumption in Apportionment Method:

“The growth of the city/town is fairly consistent with the growth of the nation.”

Steps of Apportionment Method:

1. The census population of last four to five decades is noted from the past data.
2. These census populations of city or town whose population is to be forecasted is noted down as the percentage of population of the whole country.
3. Then after, the ratio of local population to national population are calculated.
4. Graph is then plotted taking time on X-axis and the computed ratios on Y-axis.
5. The graph line is then extended smoothly to know the ratio of population during the design period.
6. The ratio thus obtained is then multiplied by the expected national population at the end of the design period. This will give the required estimated population of the city.

Expected national population data is generally available as it is released by the Census Department following a reliable specific method of population forecasting considering the situation.

Where Apportionment Method can be Used?

Ratio method is extremely suitable for cities exhibiting parallel growth as that of the nation.

But the abnormal conditions present in the local areas are totally ignored in this method, if any.

Apportionment Method Formula

Pr = population of the region

Pr’ = future population of the region

Pn = population of the nation

Pn’ = future population of the nation (obtained from census records of the country)

Graph is plotted for the city Pr/Pn as Y-axis and time as X-axis.

Then, the ratio for design period whose population is to be known is obtained from the graph – K.

This ratio is then equated to Pr’/Pn’.

K = Pr’/Pn’                                

Thus, Pr’ = K. Pn’