University of Bahr el Ghazal

College of Medicine and Health Sciences

Department of Community Medicine

DEMOGRAPHY
Introduction
 The word demography has been coined from the Greek words “Demos” and “Graphy”
which means Population and Science. As such demography is a science of studying
population and related aspects
 Demography is the scientific study of human populations.
 The three components of demographic change are births, deaths and migration. In other
words, during a period of time, population may change due to four factors, birth of a
person may take place in the area, a resident in the area may die, an outsider may move
into the area and a resident may move out of the area.
 It is mainly concerned with
1. Size: It refers to the total number of persons in the given population.
2. Distribution: It refers to the arrangement of entire population with respect to the
geographical areas at a given point of time.
3. Structure: It refers to the distribution of the given population with respect to age and sex.
4. Change: It refers to the increase or decrease in the size of the given population due to
fertility, mortality and migration.
5. Development: It refers to the development of the given population with respect to socio
economic aspects. Other characteristic like genetic inheritance, intelligence and health.

The following are number of the applications of demography in health related fields. The
knowledge in demography is helpful to public health administrators for various purposes:
 Mortality rates by age-sex and its geographical distribution with respect to various
diseases are helpful in locating and identifying diseases
 Percentage distribution of population by age-sex-location are helpful in understanding
health and health care needs of various age groups by sex by location, for planning,
designing, evaluation and effective implementation of various public health programs.
For example: Vaccination and immunization program for children under 5 years of age,
Mother and Child Health program for mother and new born, Family planning program,
old age program, nutritional program etc.
 Determining the success or failure of health programs.
 To describe the level of community health.
  To determine the leading causes of mortality and morbidity.
 To determine the relative importance of different fatal diseases with respective to age
and sex.
 To discover solution to health problems and find clues for public health administration.

Stages of Demographic Evolution

From the study of the population changes in the past and in the light of the variables
affecting the population change, five possibilities for the stages of population change can be
considered.
i. When both birth and death rates are high and are almost equal, the net resultant
change in the population neutralizes and the total population tends to be almost
stationary; this stage is termed as high stationary stage.
ii. With the advancements in medical and health facilities, the death rate begins to fall.
However with the prevailing customs and norms not permitting adoption of birth
control measures, the birth rate will be either increasing or stationary thus resulting
in a population explosion. This stage is known as early expanding stage.

iii. With further developments in medical and health technology and awareness amongst
the population about ill effects of population explosion, the death rate falls steeply
and the birth rate tends to come down slowly when the population increase may slow
down, but still the growth rate remaining at a higher level. This stage is known as
late expanding stage.

iv. With the awareness of population explosion created, the birth rate tends to decrease
and the death rate already having reached its lowest, the net resultant increase in
population may be neutralized when the population tends to be stationary. This stage
is known as low stationary stage.

v. With excessive use of birth control measures, the birth rate may fall below that of
death rate as certain amount of deaths are imminent and death rate cannot be brought
down below a certain level. At this stage, the population may start declining. This
stage is known as declining stage.
SOURCES OF DEMOGRAPHIC DATA

1. Census
2. Vital Events Registers
3. Surveys
4. Sample Registration System

Measures of Population Projection

 By “Population projections”, we mean estimating and forecasting the population of a

country or a region for a given time.
 There are mainly three types of population estimates namely
I. Inter-censal (during any two consecutive census period),
II. Post-censal (any period following latest census up to the present moment of time)
III. Future (any period time after the present moment).

The population of any area can be obtained either by the census report or through population
estimates for inter-census periods. Different methods of calculation of inter-census population
estimates are as follows;
1. Natural Increase Method:
 In this case, the number of births, deaths, immigration and emigrations are taken into
account for estimating the population during any year.
 Thus estimated population at a period = Population during the preceding census + No.
of (births - deaths) during the period after preceding census + No. of (immigrants -
emigrants) during the period after the preceding census

2. Arithmetical Progression Method:

In this method, it is assumed that the population increase is in an arithmetic
progression from year to year or between two census periods.
In other words, the rate of increase of population from one year to another is constant
and is same from year to year between two census periods.
To estimate the population during any year, the rate of increase is calculated from the
previous two census populations. This annual rate of increase is used in estimating the
population at any time from the following formula:
P t= P o + r t
Where Pt is the population at a period t years after the preceding census, r is the annual
rate of increase of population which is obtained as (1/10) of the difference between the
 population estimates at the previous two successive census and Po is the population
estimate at the immediate preceding census.

Example 1:
The calculation of population estimate of city A for the year 2007 by the above method is
illustrated below.
Population of city A in 1991: 4,130,288
Population of city A in 2001: 5,686,844
Population increase in 10 years = 1556556
Increase for one year = 155656
Increase in population for 6 years from 2001 to 2007, Census reference period (155656
x 6) = 933936
Population of city A on Census reference period of 2007
= (5,686,844 + 933936)
= 6,620,780
Similarly population estimate at any reference period can be calculated, taking the duration
from the previous Census period.