Population prediction is a very important aspect in environmental engineering that helps in determination of certain factors that helps in future planning and for accurate determination of the certain requirement in future.

The current population of any society can be estimated with census where as future predictions can be obtained with trends and knowledge of the city and its environment along with certain reasons that can input into the population like development, expansion etc.

## Definition of Population Projection

Population Projection is extrapolation of historical data into the future. An attempt to describe what is likely to happen under certain explicit assumptions about the future as related to the immediate paste.

A set of calculations, which show the future course of fertility, mortality and migration depending on the assumptions used.

Following are some of the Methods that are usually used for prediction of population;

a) Arithmetical Progression Method

b) Geometrical Progression Method

c) Annual Rate of Increase Method

d) Incremental Increase Method

e) Changing Rate of Increase Method

f) Graphical Method

Among all these types the most accurate method is that of Graphical method.

I will describe all these methods one by one;

## Arithmetic Progression Method;

In this method we assume that growth rate for certain duration of time remains constant i.e. if annual increment in rate is 20% in 1960 it will remain 20% in 1980 as well.

In this method we assume a straight line relationship between population and time i.e. population varies linearly with time. Thus we can express

P(t) = P + b*t

Where P(t) is population t years later

b is the annual amount of population change

t is time may be years

## Geometrical Progression Method

In this method the growth is assumed to follow geometric series. Growth is assumed to be constant for a specific period of time. Thus, population at the end of n years is given as;

Pn = P( 1+ i/100) n where

i = per year percentage rate of increase.

## Incremental Increase Method

Incremental increase method is similar to Arithmetic Progression method with the difference that we found out the average in the increase of population and that average is added into the net increment of population.

## Changing Rate of Increase Method

This method is similar to Geometric Progression method except that a changing rather than a constant rate of increase is assumed.

## Annual Rate of Increase Method

In this method, the rate of increase for one year is first determined and then population is predicted from there.

Pn = P (1+ i)^n

Where

Pn = population at the end of n years

P = population at any time

i = annual rate of increase of population

## Graphical Method

These mostly involve extension of the plotted data on a population-time curve. Considering towns which were in similar situations over 30 or 40 years ago and drawing graphs of their increase of population, the extension of plotted data for the city under considering can then be reasonably assumed. This method being logically based gives quite accurate prediction of population and is therefore frequently used when population figures of other similar cities are known.

## Example

Here is a simple example of Population Projection

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