Method of Population Estimation and Projection

Population census is essential for resource allocation and planning. We need alternative methods to assess the total member of population in each year because population census is carried out in every ten years.

Population Estimation

Using population estimation we can provide the total number of population for years between two censuses. It is also possible to estimate number of total population the most recent years after a census. For example, we know the number of total population the most recent years after census. We know the total number of population in Bangladesh in 2011 from population census. What about 2012, 213, 2014?

Population estimates use censuses as a baseline add births, subtract birth and make allowances for migration. They can be used for national and local planning. Population estimates are produced annually. The key point here is that population estimation are done for a short term.

Population Projection:

Population projection is an important part of population studies. However government needs to know the total number of population for long term. Such as in 25 years or 50 years from now. This is particularly important for long term planning. Population projection aim to estimate characteristics and sizes of population in the future. For example in 2015, 2050?

According to Ram and Pathak “A population projection may be defined as the set of assumption regarding future trends in fertility, mortality and migration."

Key Assumption in Population Projection

There are three major assumption of population projection

1. Fertility rate.
2. Mortality rate.
3. Migration rate.

Methods of Population Projection

The population of a geographical area grows or declines through fertility, mortality and migration.

To project population size at a future date, demographers make assumption about levels of fertility and mortality about how many people will move into or out of an area before that data. The net population increase or decrease over the period is added to the base line begging population to project future population.

There are two types of methods that are frequently used in population projection are

1. Component Method
2. Cohort Method

Component Method: In component method population projection is arrived at by using the following balance equation.

P_(b+1)+P_b+B_(b+1)- D_(b+1)+M_(b+1)

where,P_b=number of base year population.e.g.,2012

P_(b+1)=number of projected population 2050

B_(b+1)=Total births during the interval

D_(b+1)=Total death during the interval

M_(b+1)=Net migration during the interval

Cohot Component Method: Initial populations for countries or regions are grouped into chorts defined by age and sex and the projection proceeds by updating the population of each age and sex specific group according to assumptions about three components of population change, fertility, mortality and migration. Each Cohort survives to word to the next age group according to assumed age specific mortality.

Population projection may be prepared using either, subjective or objective method

Subjective Method: Subjective method is those in which data, techniques, and assumption are not clearly identified consequently, other analysis cannot replicate them exactly.

Objective Method: objective methods are those for which data, technique, and assumption are clearly identified, such that other analysts can replicate tem exactly. Following smith et al. 2001 we classify objective methods into three broad categories

1. Trend extrapolation.
2. Cohort component and 
3. Structural models.