Demography is the scientific study of human populations, particularly their size, composition, geographical distribution, and change over time. Its central processes are fertility, mortality, and migration, which determine how populations grow, shrink, and change their age structure. Demographers use statistics to measure these processes and investigate their relationships with social conditions. The field also provides estimates and projections used in public administration, research, and business planning. (iussp.org)
Scope and population dynamics
A distinction is often drawn between formal demography, concerned with population measurement and mathematical relationships, and broader population studies, concerned with the causes and consequences of demographic change. The latter connects with sociology, economics, and public health. Subjects include family formation, marriage, household composition, and differences among population groups. These approaches overlap: explaining population change requires both reliable measurement and knowledge of the circumstances surrounding demographic behaviour. (iussp.org)
For a geographically defined population, change follows the demographic balancing equation:
where is the initial population, births, deaths, immigration, and emigration during the interval. Births minus deaths constitute natural increase or decrease; immigration minus emigration constitutes net migration. The equation is an accounting identity rather than an explanation of why events occur. All components must refer to the same territory, population definition, and period. (census.gov)
Population structure influences these components. Birth numbers depend not only on fertility rates but also on the number of people at reproductive ages. Death numbers similarly reflect both mortality conditions and population composition. Consequently, populations with identical total sizes can follow different trajectories because their age distributions differ. (census.gov)
Data sources and measurement
The principal data sources are the population census, civil registration, sample surveys, population registers, and other administrative records. A census establishes a population count and its characteristics at a reference date. Civil registration records events such as births and deaths continuously, while surveys can collect detailed reproductive histories, previous residence, and family information. The United Nations combines these types of evidence in producing internationally comparable demographic estimates. (population.un.org)
Each source has limitations. Censuses may omit people or count them more than once; registration may be incomplete; respondents may misremember dates or ages. Demographers compare independent sources, assess consistency, and adjust estimates where justified. When direct records are inadequate, indirect estimation methods use information such as children ever born and children surviving to infer demographic conditions. Such estimates depend on assumptions as well as observations. (census.gov)
Demographic rates relate events to the population exposed to them. Crude birth and death rates commonly express annual events per thousand inhabitants. Age-specific rates restrict the denominator to a relevant age group, allowing fertility or mortality patterns to be examined more precisely than aggregate rates permit. (data.un.org)
Periods, cohorts, and demographic indicators
Demography distinguishes period analysis from cohort analysis. A period measure describes events during a particular interval. A cohort measure follows people sharing an event or starting point, such as birth in the same year, through time. Age, calendar period, and cohort therefore provide different ways to organize demographic experience. (iussp.org)
The total fertility rate summarizes age-specific fertility rates. It represents the number of children a hypothetical woman would have if she experienced a given period’s fertility rates throughout her reproductive years, without mortality. It is not necessarily the completed family size of an actual birth cohort: subsequent fertility patterns may differ from those observed in that period. (who.int)
A life table describes survival and death across ages under specified mortality conditions. Life expectancy at birth summarizes the average lifespan implied by those conditions. Period life expectancy applies one period’s age-specific mortality rates throughout a hypothetical lifetime; it is therefore not a direct prediction of how long children born that year will actually live. (cdc.gov)
Demographic transition and age structure
The demographic transition describes the historical shift from high fertility and mortality to low levels of both. Mortality decline commonly precedes fertility decline, creating an interval of faster population growth. Subsequent fertility reduction slows growth and changes the distribution of population across ages. The timing and trajectories of this transition vary among societies. (un.org)
Population ageing is a major consequence of sustained changes in fertility and survival. Migration can also alter age composition. Population momentum describes the influence of an existing age structure on future growth: a population with large younger generations may continue growing after fertility has fallen because many people are entering reproductive ages. Population size thus reflects past demographic events as well as present rates. (digitallibrary.un.org)
Projections, applications, and confidentiality
Population projections commonly use the cohort-component method. Starting with a population classified by age and sex, it advances cohorts through time, applies survival rates, adds births using age-specific fertility rates, and incorporates migration. Alternative assumptions produce different scenarios; probabilistic approaches express uncertainty through distributions of possible outcomes. Projections are conditional calculations, not guaranteed forecasts. (population.un.org)
Demographic evidence supports planning for education, health services, labour supply, housing, and financial transfer systems. Because underlying records concern individuals and families, data privacy is an important part of statistical practice. Published tables may use suppression, broader categories, or added statistical noise to reduce disclosure risks while retaining analytical usefulness. (digitallibrary.un.org)