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{{Short description|Science that deals with populations and their structures, statistically and theoretically}}
{{About|the discipline|the journal|Demography (journal)|the album by 16 Volt|Demography: The Basement Tapes}}
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[[File:Global Population-Pyramid-1950-to-2100.jpg|alt=The Demography of the World Population from 1950 to 2100. Data source: United Nations — World Population Prospects 2017|thumb|350x350px|The Demography of the World Population from 1950 to 2100. Data source: United Nations — World Population Prospects 2017]]

'''Demography''' ({{etymology|grc|''{{wikt-lang|grc|δῆμος}}'' ({{grc-transl|δῆμος}})|people, society||''{{wikt-lang|grc|-γραφία}}'' ({{grc-transl|-γραφία}})|writing, drawing, description}})<ref>{{cite Merriam-Webster|demography}}</ref> is the [[statistics|statistical study]] of human [[population]]s: their size, composition (e.g., ethnic group, age), and how they change through the interplay of fertility (births), mortality (deaths), and migration.<ref>McFalls Jr, Joseph A. "Population: A Lively Introduction." ''Population Bulletin'' 46.2 (1991): n2. https://www.prb.org/resources/population-a-lively-introduction/</ref>

'''Demographic analysis''' examines and measures the dimensions and [[Population dynamics|dynamics]] of populations; it can cover whole societies or groups defined by criteria such as [[education]], [[nationality]], [[religion]], and [[ethnicity]]. Educational institutions<ref>{{cite web|title=The Science of Population |url=http://demographicpartitions.org/science-population-determines-population-change/ |website=demographicpartitions.org |access-date=4 August 2015 |url-status=dead |archive-url=https://web.archive.org/web/20150814023915/http://demographicpartitions.org/science-population-determines-population-change/ |archive-date=14 August 2015 |df=dmy }}</ref> usually treat demography as a field of [[sociology]], though there are a number of independent demography departments.<ref name="ub">{{Cite web | title=UC Berkeley Demography department website | url=http://www.demog.berkeley.edu/ | access-date=12 October 2006 | archive-date=1 September 2006 | archive-url=https://web.archive.org/web/20060901191409/http://www.demog.berkeley.edu/ | url-status=live }}</ref> These methods have primarily been developed to study human populations, but are extended to a variety of areas where researchers want to know how populations of [[Social actions|social actors]] can change across time through processes of birth, death, and [[Human migration|migration]].   In the context of [[human]] biological populations, demographic analysis uses [[Public records|administrative records]] to develop an independent [[Approximation|estimate]] of the population.<ref>{{Cite web |title=Demographic Analysis |url=https://govinfo.library.unt.edu/cmb/cmbp/downloads/da.pdf |access-date=3 October 2023 |website=U.S. Census Monitoring Board}}</ref> [[Demographic]] analysis estimates are often considered a reliable standard for judging the accuracy of the [[census]] information gathered at any time.  In the [[labor force]], demographic analysis is used to estimate sizes and flows of populations of workers; in [[population ecology]] the focus is on the birth, death, migration and immigration of individuals in a population of living organisms, alternatively, in social human sciences could involve movement of firms and [[institution]]al forms. Demographic analysis is used in a wide variety of contexts. For example, it is often used in business plans, to describe the population connected to the geographic location of the business.<ref>{{cite web|url=http://biztaxlaw.about.com/od/glossaryd/a/demographics.htm|title=How to Use Demographics for Business Advertising|author=Jean Murray|work=About.com Money|accessdate=26 March 2016|archive-date=6 October 2011|archive-url=https://web.archive.org/web/20111006194323/http://biztaxlaw.about.com/od/glossaryd/a/demographics.htm|url-status=dead}}</ref> Demographic analysis is usually abbreviated as DA.<ref name="census.gov">{{cite web|url=https://www.census.gov/coverage_measurement/demographic_analysis/|title=Coverage Measurement|author=US Census Bureau Webdesign: SSD, Laura K Yax, Content: DSSD, Phil Gbur, POP, Jason Devine|publisher=|accessdate=26 March 2016|archive-date=10 January 2016|archive-url=https://web.archive.org/web/20160110142138/http://www.census.gov/coverage_measurement/demographic_analysis/|url-status=live}}</ref> For the 2010 U.S. Census, The [[United States Census Bureau|U.S. Census Bureau]] has expanded its DA categories.<ref name="census.gov"/> Also as part of the 2010 U.S. Census, DA now also includes comparative analysis between independent housing estimates, and census address lists at different key time points.<ref name="census.gov"/>

'''Patient demographics''' form the core of the data for any medical institution, such as patient and emergency contact information and patient medical record data. They allow for the identification of a patient and their categorization into categories for the purpose of statistical analysis. Patient demographics include: [[date of birth]], [[gender]], date of [[death]], postal code, ethnicity, [[blood type]], emergency contact information, family doctor, insurance provider data, [[allergies]], major diagnoses and major medical history.<ref name="What Are Patient Demographics?">{{Cite web| title=What Are Patient Demographics?| date=21 December 2011| url=https://www.macadamian.com/learn/patient-demographics/| access-date=15 November 2020| archive-date=28 January 2021| archive-url=https://web.archive.org/web/20210128074040/https://www.macadamian.com/learn/patient-demographics/| url-status=live}}</ref>

'''Formal demography''' limits its object of study to the measurement of population processes, while the broader field of '''social demography''' or population studies also analyses the relationships between economic, social, institutional, cultural, and biological processes influencing a population.<ref>Andrew Hinde ''Demographic Methods'' Ch. 1 {{ISBN|0-340-71892-7}}</ref>

==History==
Demographic thoughts traced back to antiquity, and were present in many civilisations and cultures, like [[Ancient Greece]], [[Ancient Rome]], [[China]] and [[India]].<ref name="Srivastava2006">{{cite book| url = https://books.google.com/books?id=oNcflpmW3BUC&dq=studies+in+demography&pg=PA1| title = S.C.Srivastava,''Studies in Demography'', p.39-41| isbn = 9788126119929| last1 = Srivastava| first1 = Sangya| date = December 2005| publisher = Anmol Publications Pvt. Limited}}</ref> Made up of the prefix ''[[wikt:demo-|demo-]]'' and the suffix ''[[-graphy]]'', the term ''[[wikt:demography|demography]]'' refers to the overall study of population.{{citation needed|date=May 2023}}

In ancient Greece, this can be found in the writings of [[Herodotus]], [[Thucydides]], [[Hippocrates]], [[Epicurus]], [[Protagoras]], [[Polus]], [[Plato]] and [[Aristotle]].<ref name="Srivastava2006" /> In Rome, writers and philosophers like [[Cicero]], [[Seneca the Younger|Seneca]], [[Pliny the Elder]], [[Marcus Aurelius]], [[Epictetus]], [[Cato the Elder|Cato]], and [[Columella]] also expressed important ideas on this ground.<ref name="Srivastava2006" />

In the [[Middle Ages]], [[Christians|Christian]] thinkers devoted much time in refuting the Classical ideas on demography.  Important contributors to the field were [[William of Conches]],<ref name="Biller2000">Peter Biller,''The measure of multitude: Population in medieval thought''[https://books.google.com/books?id=LnqlgqeYhwYC&dq=Nicole+Oresme+on+demographics&pg=PA312].</ref> [[Bartholomew of Lucca]],<ref name="Biller2000" /> [[William of Auvergne, Bishop of Paris|William of Auvergne]],<ref name="Biller2000" /> [[William of Pagula]],<ref name="Biller2000" /> and [[Muslims|Muslim]] sociologists like [[Ibn Khaldun]].<ref>See, e.g., [[Andrey Korotayev]], Artemy Malkov, & Daria Khaltourina (2006). [https://www.academia.edu/32757085/Introduction_to_Social_Macrodynamics._Models_of_the_World_System_Development._Moscow_KomKniga_URSS_2006 ''Introduction to Social Macrodynamics: Compact Macromodels of the World System Growth''] {{Webarchive|url=https://web.archive.org/web/20190709103050/https://www.academia.edu/32757085/Introduction_to_Social_Macrodynamics._Models_of_the_World_System_Development._Moscow_KomKniga_URSS_2006 |date=9 July 2019 }}. Moscow: URSS,  {{ISBN|5-484-00414-4}}.</ref>

One of the earliest demographic studies in the modern period was ''Natural and Political Observations Made upon the Bills of Mortality'' (1662) by [[John Graunt]], which contains a primitive form of [[actuarial table|life table]]. Among the study's findings were that one-third of the children in [[London]] died before their sixteenth birthday. Mathematicians, such as [[Edmond Halley]], developed the life table as the basis for life insurance mathematics. [[Richard Price]] was credited with the first textbook on life contingencies published in 1771,<ref>"Our Yesterdays: the History of the Actuarial Profession in North America, 1809-1979," by E.J. (Jack) Moorhead, FSA, (1/23/10 – 2/21/04), published by the Society of Actuaries as part of the profession's centennial celebration in 1989.</ref> followed later by [[Augustus De Morgan]], ''On the Application of Probabilities to Life Contingencies'' (1838).<ref>The History of Insurance, Vol 3, Edited by David Jenkins and Takau Yoneyama (1 85196 527 0): 8 Volume Set: (2000) Availability: Japan: Kinokuniya).</ref>

In 1755, [[Benjamin Franklin]] published his essay ''[[Observations Concerning the Increase of Mankind, Peopling of Countries, etc.]]'', projecting [[exponential growth]] in [[British colonies]].<ref name=valtier>{{cite journal|title="An Extravagant Assumption": The Demographic Numbers behind Benjamin Franklin's Twenty-Five-Year Doubling Period|last=von Valtier|first=William F.|journal=[[Proceedings of the American Philosophical Society]]|volume=155|number=2|date=June 2011|pages=158–188|url=http://www.amphilsoc.org/sites/default/files/proceedings/5VonValtier1550205.pdf|access-date=19 September 2018|archive-url=https://web.archive.org/web/20160305211515/https://amphilsoc.org/sites/default/files/proceedings/5VonValtier1550205.pdf|archive-date=5 March 2016|url-status=dead}}</ref> His work influenced [[Thomas Robert Malthus]],<ref>{{cite journal |last=Zirkle |first=Conway |author-link=Conway Zirkle |date=April 25, 1941 |title=Natural Selection before the 'Origin of Species' |journal=[[Proceedings of the American Philosophical Society]] |location=Philadelphia, PA |publisher=[[American Philosophical Society]] |volume=84 |issue=1 |pages=71–123 |jstor=984852 |issn=0003-049X}}</ref> who, writing at the end of the 18th century, feared that, if unchecked, population growth would tend to outstrip growth in food production, leading to ever-increasing famine and poverty (see [[Malthusian catastrophe]]). Malthus is seen as the intellectual father of ideas of [[Human overpopulation|overpopulation]] and the limits to growth. Later, more sophisticated and realistic models were presented by [[Benjamin Gompertz]] and [[Pierre François Verhulst|Verhulst]].{{citation needed|date=August 2024}}

In 1855, a [[Belgium|Belgian]] scholar Achille Guillard defined demography as the natural and social history of human species or the mathematical knowledge of populations, of their general changes, and of their physical, civil, intellectual, and moral condition.<ref>{{Cite book|title=Encyclopedia of the City|last=Caves|first=R. W.|publisher=Routledge|year=2004|pages=169}}</ref>

The period 1860–1910 can be characterized as a period of transition where in demography emerged from statistics as a separate field of interest. This period included a panoply of international 'great demographers' like [[Adolphe Quételet|Adolphe Quetelet]] (1796–1874), [[William Farr]] (1807–1883), [[Louis-Adolphe Bertillon]] (1821–1883) and his son [[Jacques Bertillon|Jacques]] (1851–1922), Joseph Körösi (1844–1906), Anders Nicolas Kaier (1838–1919), Richard Böckh (1824–1907), [[Émile Durkheim]] (1858–1917), [[Wilhelm Lexis]] (1837–1914), and [[Luigi Bodio]] (1840–1920) contributed to the development of demography and to the toolkit of methods and techniques of demographic analysis.<ref>de Gans, Henk and Frans van Poppel (2000) Contributions from the margins. Dutch statisticians, actuaries and medical doctors and the methods of demography in the time of Wilhelm Lexis. Workshop on 'Lexis in Context: German and Eastern& Northern European Contributions to Demography 1860-1910' at the Max Planck Institute for Demographic Research, Rostock, August 28 and 29, 2000.</ref>

==Methods==
[[File:1888-01-22, La Ilustración Española y Americana, El censo de población, Alcázar.jpg|thumb|Early censuses and surveys provided demographic data.]]
Demography is the [[statistical]] and mathematical study of the size, composition, and [[Population density|spatial distribution]] of human populations and how these features change over time.  Data are obtained from a census of the population and from registries: records of events like [[Birth certificate|birth]], [[death certificate|death]]s, migrations, marriages, [[divorce]]s, diseases, and [[employment]].  To do this, there needs to be an understanding of how they are calculated and the questions they answer which are included in these four concepts: [[population change]], [[standardization]] of population numbers, the demographic bookkeeping equation, and population composition.{{citation needed|date=May 2023}}

There are two types of [[data collection]]—direct and indirect—with several methods of each type.

=== Direct methods ===
Direct 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 ratio]]s 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.
[[File:Población Mundial.svg|thumb|Map of [[Country|countries]] by population]]
[[File:World population growth - time between each billion-person growth.svg|thumb|upright=1.6|Rate of human population growth showing projections for later this century<ref>{{cite journal | url = https://ourworldindata.org/world-population-growth#how-long-did-it-take-for-the-world-population-to-increase-by-one-billion | author1 = Max Roser | author1-link = Max Roser | author2 = Hannah Ritchie | author2-link = Hannah Ritchie | author3 = Esteban Ortiz-Ospina | author4 = Lucas Rodés-Guirao | title = How long did it take for the world population to increase by one billion? | journal = Our World in Data | year = 2013 | access-date = 25 November 2022 | archive-date = 13 October 2016 | archive-url = https://web.archive.org/web/20161013144559/https://ourworldindata.org/world-population-growth/#how-long-did-it-take-for-the-world-population-to-increase-by-one-billion | url-status = live }}</ref>]]

=== Indirect methods ===
Indirect 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.{{citation needed|date=May 2023}}

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]] (Hermes 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 [[National Survey of Health & Development|1946 National Survey of Health and Development]], the 1958 [[National Child Development Study]],<ref>{{cite journal |author=Power C and Elliott J |title=Cohort profile: 1958 British Cohort Study |journal=International Journal of Epidemiology |volume=35 |issue=1 |pages=34–41 |year=2006 |pmid=16155052 |doi=10.1093/ije/dyi183|doi-access=free }}</ref> the [[1970 British Cohort Study]],<ref>{{cite journal |author=Elliott J and Shepherd P|title=Cohort profile: 1970 British Birth Cohort (BCS70)|journal=International Journal of Epidemiology |volume=35 |issue=4 |pages=836–43|year=2006 |doi=10.1093/ije/dyl174 |pmid=16931528|doi-access=free }}</ref> 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.<ref>The last three are run by the [http://www.cls.ioe.ac.uk Centre for Longitudinal Studies] {{Webarchive|url=https://web.archive.org/web/20181028075855/http://www.cls.ioe.ac.uk/ |date=28 October 2018 }}</ref>

Indirect standardization is used when a population is small enough that the number of events (births, deaths, etc.) are also small. In this case, methods must be used to produce a standardized [[mortality rate]] (SMR) or standardized incidence rate (SIR).<ref>{{cite web |url=http://www.geo.hunter.cuny.edu/~imiyares/standard.htm |title=Direct and Indirect Standardization of Mortality Rates |accessdate=26 March 2016 |url-status=dead |archiveurl=https://web.archive.org/web/20160403114527/http://www.geo.hunter.cuny.edu/~imiyares/standard.htm |archivedate=3 April 2016 }}</ref><ref>{{Cite web |url=http://www.cid.harvard.edu/cidstudents/thesis_prize/thesis_2001/appendix_a.pdf |title=examples of standardization |access-date=16 October 2022 |archive-date=7 July 2017 |archive-url=https://web.archive.org/web/20170707210711/http://www.cid.harvard.edu/cidstudents/thesis_prize/thesis_2001/appendix_a.pdf |url-status=live }}</ref>

==Population change==
Population change is analyzed by measuring the change between one population size to another. [[Global population]] continues to rise, which makes population change an essential component to demographics. This is calculated by taking one population size minus the population size in an earlier [[census]].  The best way of measuring population change is using the intercensal percentage change.  The intercensal percentage change is the absolute change in population between the censuses divided by the population size in the earlier census. Next, multiply this a hundredfold to receive a [[percentage]].  When this statistic is achieved, the [[population growth]] between two or more [[nation]]s that differ in size, can be accurately measured and examined.<ref>{{Cite web |url=https://www.census.gov/prod/2004pubs/wp02-1.pdf |title=Global Population at a Glance: 2002 and Beyond |access-date=16 October 2022 |archive-date=6 January 2022 |archive-url=https://web.archive.org/web/20220106210322/https://www.census.gov/prod/2004pubs/wp02-1.pdf |url-status=live }}</ref><ref>{{Cite web |title=Global Population Profile: 2002 |url=https://www.census.gov/prod/2004pubs/wp-02.pdf |url-status=dead |archive-url=https://web.archive.org/web/20220201180245/https://www.census.gov/prod/2004pubs/wp-02.pdf |archive-date=2022-02-01 |access-date=2023-10-03 |website=[[census.gov]]}}</ref>

==Standardization of population numbers==
For there to be a [[Statistical significance|significant]] comparison, numbers must be altered for the size of the population that is under study. For example, the [[fertility rate]] is calculated as the ratio of the number of births to women of childbearing age to the total number of women in this age range. If these adjustments were not made, we would not know if a nation with a higher rate of births or deaths has a population with more women of childbearing age or more births per eligible woman.{{citation needed|date=May 2023}}

Within the category of [[standardization]], there are two major approaches: direct standardization and indirect standardization.<ref>{{Cite journal |last=Naing |first=N. N. |date=January 2000 |title=Easy way to learn standardization : direct and indirect methods |journal=The Malaysian Journal of Medical Sciences: MJMS |volume=7 |issue=1 |pages=10–15 |issn=1394-195X |pmc=3406211 |pmid=22844209}}</ref>

==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|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.<ref name="Ela 2008">Introduction to environmental engineering and science by Masters and Ela, 2008, Pearson Education, chapter 3</ref>
* 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 rate|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.<ref name="Ela 2008"/>
* A '''stationary population''', one that is both stable and unchanging in size (the difference between crude birth rate and crude death rate is zero).<ref name="Ela 2008"/>
* Measures of centralisation are concerned with the extent to which an area's population is concentrated in its [[urban area|urban centres]].<ref>Hoyt, H., [https://www.jstor.org/stable/2769393 Forces of Urban Centralization and Decentralization], [[American Journal of Sociology]], Vol. 46, No. 6 (May, 1941), pp. 843-852, accessed 2 July 2023</ref><ref>Cooper-Douglas, E., [https://www.abc.net.au/news/2023-01-06/population-outside-hobart-means-struggle-for-services/101829418 Tasmania forecast to have 79,000 more residents by 2033, with most living outside Greater Hobart], ''[[ABC News (Australia)|ABC News]]'', published 5 January 2023, accessed 2 July 2023</ref>

A stable population does not necessarily remain fixed in size. It can be expanding or shrinking.<ref name="Ela 2008"/>

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 in 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 population==
Suppose that a country (or other entity) contains ''Population<sub>t</sub>'' persons at time ''t''.
What is the size of the population at time ''t'' + 1 ?

:<math>\text{Population}_{t+1} = \text{Population}_t + \text{Natural Increase}_t + \text{Net Migration}_t</math>

Natural increase from time ''t'' to ''t'' + 1:

:<math>\text{Natural Increase}_t = \text{Births}_t - \text{Deaths}_t</math>

[[Net migration rate|Net migration]] from time ''t'' to ''t'' + 1:

:<math>\text{Net Migration}_t = \text{Immigration}_t - \text{Emigration}_t</math>

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.<ref>See, for example, Barbara A. Anderson and Brian D. Silver, "Estimating Russification of Ethnic Identity Among Non-Russians in the USSR," ''Demography'', Vol. 20, No. 4 (Nov., 1983): 461-489.</ref>

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 ([[United Nations]]) [[World Health Organization|WHO]] 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&nbsp;billion by 2075. Working independently, demographers at the [[International Institute for Applied Systems Analysis]] in [[Austria]] expect world population to peak at 9&nbsp;billion by 2070.<ref>{{cite journal|url=http://ace1.ma.utexas.edu/users/davis/375/reading/worldbirthrate.pdf |title=Doubling of world population unlikely |first=Wolfgang |last=Lutz |author2=Sanderson, Warren |author3=Scherbov, Sergei |journal=Nature |volume=387 |pages=803–805 |date=1997-06-19 |access-date=2008-11-13 |doi=10.1038/42935 |pmid=9194559 |issue=6635 |bibcode=1997Natur.387..803L |s2cid=4306159 |url-status=dead |archive-url=https://web.archive.org/web/20081216230409/http://ace1.ma.utexas.edu/users/davis/375/reading/worldbirthrate.pdf |archive-date=16 December 2008 |df=dmy }}</ref> Throughout the 21st century, the average age of the population is likely to continue to rise.

===Science of population===
Populations 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).<ref>[[John Bongaarts]]. The Fertility-Inhibiting Effects of the Intermediate Fertility Variables. Studies in Family Planning, Vol. 13, No. 6/7. (Jun. - Jul., 1982), pp. 179-189.</ref> 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|life table]], a statistical device that provides information about the mortality conditions (most notably the life expectancy) in the population.<ref>{{cite web| url = https://www.cdc.gov/nchs/products/pubs/pubd/lftbls/lftbls.htm| title = N C H S - Life Tables<!-- Bot generated title -->| access-date = 9 September 2017| archive-date = 29 July 2020| archive-url = https://web.archive.org/web/20200729200625/https://www.cdc.gov/nchs/products/pubs/pubd/lftbls/lftbls.htm| url-status = live}}</ref>

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.<ref>Donald T. Rowland ''Demographic Methods and Concepts'' Ch. 11 {{ISBN|0-19-875263-6}}</ref>

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 [[Committee for International Cooperation in National Research in Demography|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]],<ref>{{cite web| url = http://www.iussp.org/| title = International Union for the Scientific Study of Population| access-date = 20 April 2008| archive-date = 6 September 2019| archive-url = https://web.archive.org/web/20190906221242/https://iussp.org/| url-status = live}}</ref> or a national association such as the [[Population Association of America]] in the United States,<ref>{{cite web| url = http://www.populationassociation.org/| title = Population Association of America| access-date = 14 April 2011| archive-date = 19 March 2011| archive-url = https://web.archive.org/web/20110319153140/http://www.populationassociation.org/| url-status = live}}</ref> or affiliates of the Federation of Canadian Demographers in [[Canada]].<ref>{{Cite web |url=https://fcdweb.wordpress.com/ |title=Fédération canadienne de démographie – Federation of Canadian Demographers |access-date=7 December 2022 |archive-date=7 February 2023 |archive-url=https://web.archive.org/web/20230207210924/https://fcdweb.wordpress.com/ |url-status=live }}</ref>

==Population composition==
[[File:Population by broad age group projected to 2100, OWID.svg|thumb|upright=1.4|World demography by age composition from 1950 to 2100 (projected).<ref name="q917">{{cite web | title=Population by age group | website=Our World in Data | url=https://ourworldindata.org/grapher/population-by-age-group | access-date=26 January 2025}}</ref>]]
'''Population composition''' is the description of population defined by characteristics such as age, [[Race (classification of humans)|race]], sex or [[marital status]].  These descriptions can be necessary for understanding the [[social dynamics]] from historical and comparative research.  This data is often compared using a [[population pyramid]].

Population composition is also a very important part of historical research.  Information ranging back hundreds of years is not always worthwhile, because the numbers of people for which data are available may not provide the information that is important (such as [[population size]]).  Lack of information on the original data-collection procedures may prevent accurate evaluation of data quality.

==Demographic analysis in institutions and organizations==

===Labor market===
The demographic analysis of [[labor market]]s can be used to show slow population growth, [[population ageing]], and the increased importance of immigration.  The U.S. Census Bureau projects that in the next 100 years, the United States will face some dramatic demographic changes.{{Citation needed|date=March 2015}}  The population is expected to grow more slowly and age more rapidly than ever before and the nation will become a nation of immigrants.  This influx is projected to rise over the next century as new immigrants and their children will account for over half the U.S. population.  These demographic shifts could ignite major adjustments in the economy, more specifically, in labor markets.{{Citation needed|date=February 2011}}

===Turnover and in internal labor markets===
People decide to exit organizations for many reasons, such as, better jobs, dissatisfaction, and concerns within the family.  The causes of turnover can be split into two separate factors, one linked with the culture of the organization, and the other relating to all other factors.  People who do not fully accept a culture might leave voluntarily.  Or, some individuals might leave because they fail to fit in and fail to change within a particular organization.

===Population ecology of organizations===

{{main|organizational ecology}}

A basic definition of population ecology is a study of the distribution and abundance of organisms.  As it relates to organizations and demography, organizations go through various [[Public liability|liabilities]] to their continued survival.  Hospitals, like all other large and complex organizations are impacted in the environment they work. For example, a study was done on the closure of acute care hospitals in Florida between a particular time.  The study examined effect size, age, and niche density of these particular hospitals. A population theory says that organizational outcomes are mostly determined by [[environmental factor]]s.  Among several factors of the theory, there are four that apply to the hospital closure example: size, age, density of niches in which organizations operate, and density of niches in which organizations are established.{{citation needed|date=May 2023}}

====Business organizations====
Problems in which demographers may be called upon to assist business organizations are when determining the best prospective location in an area of a branch store or service outlet, predicting the demand for a new product, and to analyze certain dynamics of a company's workforce.  Choosing a new location for a branch of a bank, choosing the area in which to start a new supermarket, consulting a bank [[loan officer]] that a particular location would be a beneficial site to start a car wash, and determining what shopping area would be best to buy and be redeveloped in metropolis area are types of problems in which demographers can be called upon.

Standardization is a useful demographic technique used in the analysis of a business.  It can be used as an interpretive and analytic tool for the comparison of different markets.

====Nonprofit organizations====
These organizations have interests about the number and characteristics of their clients so they can maximize the sale of their products, their outlook on their influence, or the ends of their power, services, and beneficial works.

==See also==
{{div col}}
* [[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]]''
{{div col end}}

== References ==
{{reflist}}

==Further reading==
* [[Josef Ehmer]], Jens Ehrhardt, Martin Kohli (Eds.): [https://web.archive.org/web/20130527141349/http://www.gesis.org/en/hsr/current-issues/current-issues-2010-2012/362-fertility/ ''Fertility in the History of the 20th Century: Trends, Theories, Policies, Discourses''].  [[Historical Social Research]] 36 (2), 2011.
* Glad, John. 2008. ''[http://www.whatwemaybe.org/txt/txt0000/Glad.John.2008.FHE.Meisenberg-abridgement.en.pdf Future Human Evolution: Eugenics in the Twenty-First Century]''. Hermitage Publishers, {{ISBN|1-55779-154-6}}
* Gavrilova N.S., Gavrilov L.A. 2011. Ageing and Longevity: Mortality Laws and Mortality Forecasts for Ageing Populations [In Czech: Stárnutí a dlouhověkost: Zákony a prognózy úmrtnosti pro stárnoucí populace]. Demografie,  53(2): 109–128.
* Preston, Samuel, Patrick Heuveline, and Michel Guillot. 2000. ''Demography: Measuring and Modeling Population Processes''. Blackwell Publishing.
* Gavrilov L.A., Gavrilova N.S.  2010. Demographic Consequences of Defeating Aging.  Rejuvenation Research, 13(2-3): 329–334.
* [[Paul R. Ehrlich]] (1968), ''[[The Population Bomb]]'' Controversial Neo-Malthusianist pamphlet
* Leonid A. Gavrilov & Natalia S. Gavrilova (1991), ''The Biology of Life Span: A Quantitative Approach''. New York: Harwood Academic Publisher, {{ISBN|3-7186-4983-7}}
* [[Andrey Korotayev]] & [[Daria Khaltourina]] (2006). [https://www.academia.edu/35443515/Introduction_to_Social_Macrodynamics_Compact_Macromodels_of_the_World_System_Growth._Moscow_KomKniga_2006 ''Introduction to Social Macrodynamics: Compact Macromodels of the World System Growth''. Moscow: URSS]  {{ISBN|5-484-00414-4}} [http://urss.ru/cgi-bin/db.pl?cp=&page=Book&id=34250&lang=en&blang=en&list=14]
* Uhlenberg P. (Editor), (2009) International Handbook of the Demography of Aging, New York:  Springer-Verlag, pp.&nbsp;113–131.
* Paul Demeny and Geoffrey McNicoll (Eds.). 2003. The Encyclopedia of Population. New York, Macmillan Reference USA, vol.1, 32-37
* [[Phillip Longman]] (2004), ''The Empty Cradle: how falling birth rates threaten global prosperity and what to do about it''
* Sven Kunisch, Stephan A. Boehm, Michael Boppel (eds) (2011). ''From Grey to Silver: Managing the Demographic Change Successfully'', Springer-Verlag, Berlin Heidelberg, {{ISBN|978-3-642-15593-2}}
* [[Joe McFalls]]  (2007), ''Population: A Lively Introduction,'' [[Population Reference Bureau]] [http://prb.org/Publications/PopulationBulletins/2007/PopulationALivelyIntroduction.aspx] {{Webarchive|url=https://web.archive.org/web/20130601045452/http://www.prb.org/Publications/PopulationBulletins/2007/PopulationALivelyIntroduction.aspx |date=1 June 2013 }}
* [[Ben J. Wattenberg]] (2004), ''How the New Demography of Depopulation Will Shape Our Future''. Chicago: R. Dee, {{ISBN|1-56663-606-X}}
* Perry, Marc J. & Mackun, Paul J. ''Population Change & Distribution: Census 2000 Brief''. (2001)
* Preston, Samuel; Heuveline, Patrick; and Guillot Michel. 2000. ''Demography: Measuring and Modeling Population Processes''. Blackwell Publishing.
* Schutt, Russell K. 2006. "Investigating the Social World: The Process and Practice of Research". SAGE Publications.
* [[Jacob S. Siegal|Siegal, Jacob S.]] (2002), ''Applied Demography: Applications to Business, Government, Law, and Public Policy''. San Diego: Academic Press.
* [[Ben J. Wattenberg|Wattenberg, Ben J.]] (2004), ''How the New Demography of Depopulation Will Shape Our Future''. Chicago: R. Dee, {{ISBN|1-56663-606-X}}

== External links ==
{{Wiktionary|demography}}
{{Commons category|Demography}}
{{Wikisource portal|Demography}}
* [https://web.archive.org/web/20160304072527/https://www.peeparound.com/ Quick demography data lookup] (archived 4 March 2016)
* [http://www.historicalstatistics.org/ Historicalstatistics.org] Links to historical demographic and economic statistics
* [https://www.un.org/development/desa/pd/ United Nations Population Division: Homepage]
** [https://web.archive.org/web/20110506065230/http://esa.un.org/unpd/wpp/index.htm World Population Prospects, the 2012 Revision], Population estimates and projections for 230 countries and areas (archived 6 May 2011)
** [https://www.un.org/en/development/desa/publications/world-urbanization-prospects-the-2011-revision.html World Urbanization Prospects, the 2011 Revision], Estimates and projections of urban and rural populations and urban agglomerations
** [https://web.archive.org/web/20121213035405/http://esa.un.org/unpd/ppp/index.htm Probabilistic Population Projections, the 2nd Revision], Probabilistic Population Projections, based on the 2010 Revision of the World Population Prospects (archived 13 December 2012)
* [http://www.aetheling.com/NL/sim/population/population1.html Java Simulation of Population Dynamics].
* [http://gsociology.icaap.org/basicguide.html Basic Guide to the World: Population changes and trends, 1960–2003]
* [http://gsociology.icaap.org/report/demsum.html Brief review of world basic demographic trends]
* [https://unece.org/population/fertility-and-family-survey-ffs Family and Fertility Surveys] (FFS)

{{Social sciences}}
{{Social surveys}}
{{Authority control}}

[[Category:Demography| ]]
[[Category:Actuarial science]]
[[Category:Environmental social science]]
[[Category:Interdisciplinary subfields of sociology]]
[[Category:Human geography]]
[[Category:Market segmentation]]
[[Category:Human populations]]