Demography is statistical study of human population. It can be very general science which can be applied to any type of dynamic human population, that is, one that changes over time or space. It includes the study of size, structure and distribution of the populations, and spatial and/or temporal changes in them in response to birth, migration, aging and death.
Demographic analysis can be applied to complete societies or to groups stated by criteria like education, nationality, religion and ethnicity. Institutionally, demography is generally considered field of sociology, though there are number of independent demography departments. Formal demography restricts its object of study to measurement of population's processes, while broader field of social demography population studies also study relationships between economic, social, cultural and biological processes influencing a population.
Methods of data collection:
There are two methods of data collection:
1. Direct data: It comes from fundamental statistics registries which track all births and deaths and certain changes in legal status like marriage, divorce, and migration (registration of place of residence). In developed countries with good registration systems (like United States and much of Europe), registry statistics are best method for evaluating number of births and deaths.
A census is other common direct method of collecting demographic data. The census is generally performed by the national government and tries to enumerate every person in a country. Though, on the contrary to vital statistics data that are typically collected continuously and summarized on annual basis, censuses usually occur only every 10 years or so, and therefore is not typically the best source of data on births and deaths.
In countries in which vital registration system may be incomplete, censuses are also utilized as direct source of information about fertility and mortality; for instance censuses of People's Republic of China collect information on births and deaths which occurred in 18 months immediately preceding census.
2. Indirect methods of gathering data are needed in countries where full data are not available, like is the case in much of developing world. One of these techniques is sister method, where survey researchers ask women how many of their sisters have died or had children and at what age. With these surveys, researchers can then indirectly evaluate birth or death rates for whole population. Other indirect methods comprise asking people about siblings, parents, and children.
There are varieties of demographic methods for modeling population processes. They comprise models of mortality (comprising life table, hazards models, Gompertz models, multiple decrement life tables, Cox proportional hazards models, Brass relational logits), fertility (Hernes model, Coale-Trussell models, parity progression ratios)
The crude birth rate is annual number of live births per 1,000 people.
The general fertility rate is annual number of live births per 1,000 women of childbearing age (frequently taken to be from 15 to 49 years old, but at times from 15 to 44).
An age-specific fertility rate is the annual number of live births per 1,000 women in specific age groups (generally age 15-19, 20-24 etc.)
Crude death rate is the annual number of deaths per 1,000 people.
The infant mortality rate is annual number of deaths of children less than 1 year old per 1,000 live births.
The expectation of life (or life expectancy) is number of years that an individual at given age could expect to live at present mortality levels.
Total fertility rate is the number of live births per woman completing her reproductive life, if her childbearing at every age reflected current age-specific fertility rates.
Assume that country (or other entity) has Populationt persons at time t. What is the size of population at time t + 1?
Populationt+1 = Populationt + Naturalincreaset + Netmigrationt
Natural increase from time t to t + 1:
Naturalincreaset = Birthst - Deathst
Net migration from time t to t + 1:
Netmigrationt = Immigrationt - Emigrationt
This basic equation can also be applied to subpopulations. For instance, population size of ethnic groups or nationalities inside given society or country is subject to same sources of change. Though, when dealing with ethnic groups, net migration might have to be subdivided in physical migration and ethnic reidentification (assimilation). Individuals who change the ethnic self-labels or whose ethnic classification in government statistics changes over time may be thought of as migrating or moving from one population subcategory to another.
1) Science of population:
Populations can change through three procedures: fertility, mortality, and migration.
i) Fertility involves number of children which women have and is to be contrasted with fecundity (a woman's childbearing potential).
ii) Mortality is study of causes, consequences, and measurement of procedures affecting death to members of population. Demographers most generally study mortality using Life Table, a statistical device that gives information about mortality conditions (most notably the life expectancy) in population.
iii) Migration refers to movement of persons from origin place to destination place across some pre-defined, political boundary. Migration researchers don't designate movements' migrations unless they are rather permanent. Therefore demographers don't consider tourists and travelers to be migrating.
Demography is today extensively taught in several universities across world, attracting students with early training in social sciences, statistics or health studies. Being at crossroads of numerous disciplines like sociology, epidemiology, anthropology and history, economics, geography, demography offers tools to approach large range of population issues by combining more technical quantitative approach which represents core of discipline with several other methods borrowed from social or other sciences.
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