HistoryDemographic thoughts traced back to antiquity, and were present in many civilisations and cultures, like Ancient Greece, Ancient Rome, China and India.S.C.Srivastava,''Studies in Demography'', p.39-41
MethodsThere are two types of data collection—direct and indirect—with several different methods of each type.
Direct methodsDirect data comes from vital statistics registries that track all births and deaths as well as certain changes in legal status such as marriage, divorce, and migration (registration of place of residence). In developed countries with good registration systems (such as the United States and much of Europe), registry statistics are the best method for estimating the number of births and deaths. A census is the other common direct method of collecting demographic data. A census is usually conducted by a national government and attempts to enumerate every person in a country. In contrast to vital statistics data, which are typically collected continuously and summarized on an annual basis, censuses typically occur only every 10 years or so, and thus are not usually the best source of data on births and deaths. Analyses are conducted after a census to estimate how much over or undercounting took place. These compare the sex ratios from the census data to those estimated from natural values and mortality data. Censuses do more than just count people. They typically collect information about families or households in addition to individual characteristics such as age, sex, marital status, literacy/education, employment status, and occupation, and geographical location. They may also collect data on migration (or place of birth or of previous residence), language, religion, nationality (or ethnicity or race), and citizenship. In countries in which the vital registration system may be incomplete, the censuses are also used as a direct source of information about fertility and mortality; for example the censuses of the People's Republic of China gather information on births and deaths that occurred in the 18 months immediately preceding the census.
Indirect methodsIndirect methods of collecting data are required in countries and periods where full data are not available, such as is the case in much of the developing world, and most of historical demography. One of these techniques in contemporary demography is the 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 estimate birth or death rates for the entire population. Other indirect methods in contemporary demography include asking people about siblings, parents, and children. Other indirect methods are necessary in historical demography. There are a variety of demographic methods for modelling population processes. They include models of mortality (including the life table, Gompertz curve, Gompertz models, Proportional hazards models, hazards models, Proportional hazards models, Cox proportional hazards models, Decrement table, multiple decrement life tables, Brass relational logits), fertility (Hernes model, Ansley J. Coale, Coale-Trussell models, parity progression ratios), marriage (Singulate Mean at Marriage, Page model), disability (Sullivan's method, multistate life tables), population projections (Lee–Carter model, Lee-Carter model, the Leslie matrix, Leslie Matrix), and population momentum (Nathan Keyfitz, Keyfitz). The United Kingdom has a series of four national birth cohort studies, the first three spaced apart by 12 years: the 1946 National Survey of Health and Development, the 1958 National Child Development Study, the 1970 British Cohort Study, and the Millennium Cohort Study, begun much more recently in 2000. These have followed the lives of samples of people (typically beginning with around 17,000 in each study) for many years, and are still continuing. As the samples have been drawn in a nationally representative way, inferences can be drawn from these studies about the differences between four distinct generations of British people in terms of their health, education, attitudes, childbearing and employment patterns.
Common rates and ratios* The crude birth rate, the annual number of live births per 1,000 people. * The general fertility rate, the annual number of live births per 1,000 women of childbearing age (often taken to be from 15 to 49 years old, but sometimes from 15 to 44). * The age-specific fertility rates, the annual number of live births per 1,000 women in particular age groups (usually age 15–19, 20-24 etc.) * The crude death rate, the annual number of deaths per 1,000 people. * The infant mortality rate, the annual number of deaths of children less than 1 year old per 1,000 live births. * The expectation of life (or life expectancy), the number of years that an individual at a given age could expect to live at present mortality levels. * The total fertility rate, the number of live births per woman completing her reproductive life, if her childbearing at each age reflected current age-specific fertility rates. * The replacement level fertility, the average number of children women must have in order to replace the population for the next generation. For example, the replacement level fertility in the US is 2.11.Introduction to environmental engineering and science by Masters and Ela, 2008, Pearson Education, chapter 3 * The gross reproduction rate, the number of daughters who would be born to a woman completing her reproductive life at current age-specific fertility rates. * The net reproduction ratio is the expected number of daughters, per newborn prospective mother, who may or may not survive to and through the ages of childbearing. * A stable population, one that has had constant crude birth and death rates for such a long period of time that the percentage of people in every age class remains constant, or equivalently, the population pyramid has an unchanging structure. * A stationary population, one that is both stable and unchanging in size (the difference between crude birth rate and crude death rate is zero). A stable population does not necessarily remain fixed in size. It can be expanding or shrinking. Note that the crude death rate as defined above and applied to a whole population can give a misleading impression. For example, the number of deaths per 1,000 people can be higher for developed nations than in less-developed countries, despite standards of health being better in developed countries. This is because developed countries have proportionally more older people, who are more likely to die in a given year, so that the overall mortality rate can be higher even if the mortality rate at any given age is lower. A more complete picture of mortality is given by a life table, which summarizes mortality separately at each age. A life table is necessary to give a good estimate of life expectancy.
Basic equation regarding development of a populationSuppose that a country (or other entity) contains ''Populationt'' persons at time ''t''. What is the size of the population at time ''t'' + 1 ? : Natural increase from time ''t'' to ''t'' + 1: : Net migration from time ''t'' to ''t'' + 1: : These basic equations can also be applied to subpopulations. For example, the population size of ethnic groups or nationalities within a given society or country is subject to the same sources of change. When dealing with ethnic groups, however, "net migration" might have to be subdivided into physical migration and ethnic reidentification (Cultural assimilation, assimilation). Individuals who change their 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. More generally, while the basic demographic equation holds true by definition, in practice the recording and counting of events (births, deaths, immigration, emigration) and the enumeration of the total population size are subject to error. So allowance needs to be made for error in the underlying statistics when any accounting of population size or change is made. The figure in this section shows the latest (2004) UN projections of world population out to the year 2150 (red = high, orange = medium, green = low). The UN "medium" projection shows world population reaching an approximate equilibrium at 9 billion by 2075. Working independently, demographers at the International Institute for Applied Systems Analysis in Austria expect world population to peak at 9 billion by 2070. Throughout the 21st century, the average age of the population is likely to continue to rise.
Science of populationPopulations can change through three processes: fertility, mortality, and migration. Fertility involves the number of children that women have and is to be contrasted with fecundity (a woman's childbearing potential). Mortality is the study of the causes, consequences, and measurement of processes affecting death to members of the population. Demographers most commonly study mortality using the Life Table, a statistical device that provides information about the mortality conditions (most notably the life expectancy) in the population. Migration refers to the movement of persons from a locality of origin to a destination place across some predefined, political boundary. Migration researchers do not designate movements 'migrations' unless they are somewhat permanent. Thus demographers do not consider tourists and travellers to be migrating. While demographers who study migration typically do so through census data on place of residence, indirect sources of data including tax forms and labour force surveys are also important. Demography is today widely taught in many universities across the world, attracting students with initial training in social sciences, statistics or health studies. Being at the crossroads of several disciplines such as sociology, economics, epidemiology, geography, anthropology and history, demography offers tools to approach a large range of population issues by combining a more technical quantitative approach that represents the core of the discipline with many other methods borrowed from social or other sciences. Demographic research is conducted in universities, in research institutes as well as in statistical departments and in several international agencies. Population institutions are part of the Cicred (International Committee for Coordination of Demographic Research) network while most individual scientists engaged in demographic research are members of the International Union for the Scientific Study of Population, or a national association such as the Population Association of America in the United States, or affiliates of the Federation of Canadian Demographers in Canada.
See also* Biodemography *Biodemography of human longevity * Demographics of the world * Demographic economics * Gompertz–Makeham law of mortality * Linguistic demography * List of demographics articles * Medieval demography * National Security Study Memorandum 200 of 1974 * NRS social grade * Political demography * Population biology * Population dynamics * Population geography * Population reconstruction * Population statistics * Religious demography * Replacement migration * Reproductive health
Social surveys* Current Population Survey (CPS) * Demographic and Health Surveys (DHS) * European Social Survey (ESS) * General Social Survey (GSS) * German General Social Survey (ALLBUS) * Multiple Indicator Cluster Surveys (MICS) * National Longitudinal Survey (NLS) * Panel Study of Income Dynamics (PSID) * PMA2020, Performance Monitoring and Accountability 2020 (PMA2020) * Socio-Economic Panel (SOEP, German) * World Values Survey (WVS)
Organizations* Global Social Change Research Project (United States) * Institut national d'études démographiques (INED) (France) * Max Planck Institute for Demographic Research (Germany) * Office of Population Research (Princeton University) (United States) * Population Council (United States) * Population Studies Center at the University of Michigan (United States) * Vienna Institute of Demography (VID) (Austria) * Wittgenstein Centre for Demography and Global Human Capital (Austria)
Scientific journals* ''Brazilian Journal of Population Studies'' * ''Cahiers québécois de démographie'' * ''Demography (journal), Demography'' * ''Population and Development Review''
Further reading* Josef Ehmer, Jens Ehrhardt, Martin Kohli (Eds.)