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Lecture 2: Basic Concepts and Measurements
Slide 1
SIZE
As said earlier, the term size refers to what is the total number of people inhabiting an area. The area may the world as a whole, a nation, a region, a locality or a ward within the locality, or household. The number includes both males and females and all age groups.
GROWTH RATE AND DOBULING TIME
Growth rate shows percentage change in population of an area, and is expressed on per year basis. It is computed from quinquennial and decennial censuses of population. Suppose the population of a country is 100,345 in year 1991 and it has risen to 120,619 in year 2001. Then the decadal growth rate of the population is
((120,619-100,345)/100,345)* i.e., 20.20 percent. On per year basis it may be defined as 2.02 percent per year (average annual rate of growth).
In advanced technical discussions in place of simple growth rate exponential growth rate is preferred. This is computed by using the following formula:
rt
Pt = P 0 e
where Pt refers to population at time t, P 0 to population at time 0, r to rate of growth and t to time. For short time it does not matter much whether you use the decadal growth rate method or exponential growth rate method. The former assumes a discrete change in population size, the latter a continuous change.
The growth rate is often converted into doubling time which conveys that at a given rate of growth how much time would be required to double the population size.
BOX 6.1: DOUBLING TIME
Imagine that a population is growing exponentially at rate r, i.e.,
rt
Pt = P 0 e
where Pt refers to population at time t, P 0 to initial population (at t=0), e to exponential function and r to rate of growth. By taking natural logarithm on both the sides it can be shown that
0
1 log
P
P
r
t = t
Putting Pt = 2 P 0 , one gets t = .69/r For approximate values of it, doubling time t is often expressed as 70/r in which r is expressed in percentage form. The following table gives the doubling time for different values of growth rate.
r (per year) Doubling time in years
DISTRIBUTION
Distribution is a more general term than composition. It refers to any statistical classification of population according to a given characteristic. The most commonly used types of population distribution are urban-rural and spatial distributions of population. They are obtained by calculating percentage of total population living in urban areas and in different geographical regions, respectively. Thus
Percent urban = (Urban population/total population)* Percent of population living in Uttar Pradesh State = (Population of the state/total population of India)*
Changes in spatial distribution of population, i.e., percent of population living in different states or regions may be caused by difference in fertility or mortality or the process of migration – movement of people from one state or region to another for education, employment, marriage or other reasons.
As above, you may study the following:
- Occupational and industrial distribution of population
- Income and wealth distributions of population
- Distribution of population by source of drinking water and possession of household amenities
- Distribution of population by place of birth and place of current residence
Population distribution, as defined above, can be compared between different countries/societies as it makes the numbers in any category of study free from the effect of size of total population.
POPULATION PROCESSES
Study of population processes requires separate measurements of nuptiality, fertility, mortality, migration and social mobility. Among them, nuptiality is measured in terms of mean age of marriage and proportion married in different age groups. Fertility is measured by birth rate, total number of children ever born, and total fertility rate.
Birth rate is defined as ((Number of births in a year/total population))*1,
If a measure of fertility is to be obtained from census or surveys average number of children ever born per woman may be computed for different age groups of women. Average number of children ever born among women in the age group 40-44 (sometimes 45-49) is of special significance. It is called the total fertility rate.
Likewise, mortality is measured by death rate. Death rate is defined as
(Number of deaths in a year/total population) *1,
Birth and death rates are called crude rates as they are dependent not only on the rapidity of reproduction and mortality but also on age and sex composition of population. In many developed countries where on average people live longer than in the less developed countries death rates are higher because more of their population consists of old people. In less developed countries people live shorter than in the developed countries but more of their population consists of young people. Therefore their death rates are lower.
The most commonly used measure of fertility is total fertility rate. For computing it, one has to calculate age-specific-fertility-rates (ASFR) at different ages.
ASFRx = (No. of children born in a year to women aged x)/(No. of women aged x)
Slide 6
Sum of age-specific-fertility-rates over all ages (from 15 to 44 or 49) is called total fertility rate (TFR). It is interpreted as the average number of children born per woman in the entire life time by a group of
