Concepts of Demography

1) Vital rates- Vital rates refer to how fast vital statistics change in a population. There are 2 categories within vital rates: crude rates and refined rates. Crude rates measure vital statistics in a general population. Refined rates measure the change in vital statistics in a specific demographic.

 

2) Life tables- A life table is a table which shows, for a person at each age, what the probability is that they die before their next birthday. From this starting point, a number of statistics can be derived and thus also included in the table is:

 

• the probability of surviving any particular year of age
• the remaining life expectancy for people at different ages
• the proportion of the original birth cohort still alive.
• Life tables are usually constructed separately for men and for women because of their substantially different mortality rates.

 

 Construction of life tables

 

Age specific mortality rates are applied to a national population, typically of 100,000.  Starting at birth, the probability of dying in each period is applied to the number of people surviving to the beginning of the period, so that the initial figure slowly reduces to zero. The different elements required for a life table include (using standard notations):

 

 This sort of life table is based on current age-specific death rates for each age or age band used and are called period life tables and are the most frequently used type.  In contrast, actual life expectancy of a particular birth cohort can only be calculated when everyone in this cohort is dead.  This approach uses a cohort life table and requires data over many years to prepare just a single complete cohort life table.

 

3) Total fertility rate- The total fertility rate of a population is the average number of children that would be born to a woman over her lifetime if she was to experience the exact current age-specific fertility rates through her lifetime she was to live from birth until the end of her reproductive life.

 

4) Gross reproduction rate- The gross reproduction rate is the average number of daughters a woman would have if she survived all of her childbearing years, which is roughly to the age of 45, subject to the age-specific fertility rate and sex ratio at birth throughout that period.

 

5) Net reproduction rate-In population ecology and demography, the net reproduction rate, R₀, is the average number of offspring that would be born to a female if she passed through her lifetime conforming to the age-specific fertility and mortality rates of a given year

 

6) Age pyramids- Population pyramids are graphical representations of the age and sex of a population. For this reason, population pyramids are also referred to as age-sex pyramids. There are three types of population pyramids: Expansive, Constrictive, and Stationary

 

(a) Expansive population pyramids depict populations that have a larger percentage of people in younger age groups. Populations with this shape usually have high fertility rates with lower life expectancies. Many third world countries have expansive population pyramids

 

(b) Constrictive population pyramids are named so because they are constricted at the bottom. There is a lower percentage of younger people. Constrictive population pyramids show declining birth rates, since each succeeding age group is getting smaller and smaller. The United States has a constrictive population pyramid

 

(c) Stationary population pyramids are those that show a somewhat equal proportion of the population in each age group. There is not a decrease or increase in population; it is stable. Austria has a stationary population pyramid.

 

7) Population projection- A population projection gives a picture of what the future size and structure of the population by sex and age might look like.It is based on knowledge of the past trends, and, for the future, on assumptions made for three components: fertility, mortality and migration.Different evolution assumptions are made for each component, constituting different scenarios.The projections serve as a basis for long-term thinking, particularly in terms of collective development.They make it possible to analyse population trends if the assumptions are true, but are not forecasts. Individual behaviour, certain public policy actions, scientific progress or unforeseen events (weather events, epidemics) in the coming years may have a lasting effect and significantly influence trends, which the projections do not take into account.

 

8) Stable population- A population is called stable when both its growth rate and its relative age distribution do not change over time. Hence the size of a stable population grows or diminishes at a constant rate, but each age group has a constant share.

 

 9)  Stationary population- A stationary population is a special example of a stable population with a zero growth rate, neither growing nor shrinking in size, and is equivalent to a life table population. By definition, stable populations have age-specific fertility and mortality rates that remain constant over time.

 

10) Quasi-stationary population-In probability a quasi-stationary distribution is a random process that admits one or several absorbing states that are reached almost surely, but is initially distributed such that it can evolve for a long time without reaching it.