What is demography?
Demography is the scientific study of population. It is concerned with the ‘numbering of the people' and with understanding population dynamics – how populations change in response to the interplay between fertility, mortality and migration. This understanding is a pre-requisite for making the forecasts about future population size and structure which are required for much government and business planning. Demography is largely concerned with answering questions about how populations change and with the measurement of population and the components of population change. The broader field of population studies embraces questions of why these changes occur, and with what consequences, and encompasses a broad range of multidisciplinary enquiry into characteristics and behaviour of populations and population sub-groups. Much of the work of demographers and population scientists is concerned with the interplay between population change and policy and the field is very far from being a purely academic one.
Any population comprises those who have made an entry and not yet exited. When whole populations of defined geographic areas are considered, the only means of entry are birth or immigration and the only means of exit death or emigration. Of the three demographic determinants of population size, structure and growth, fertility (the child bearing performance of an individual, couple or population) has historically been of much greater importance than either mortality (death) or migration (permanent or semi-permanent residential move). Every birth represents not just an addition to the current generation of children, but also potentially an exponentially increasing augmentation in the size of future generations. Death carries no such promise of future return, although in low mortality populations with old age structures falling mortality is now the major driver of further population ageing. The third determinant – migration – is generally not of significant magnitude to have a major impact on most national populations, although there are exceptions especially when natural increase is close to zero or when migration waves are substantial. For social and biological reasons fertility, mortality and migration have interactive effects. Decreases in mortality in age groups with reproductive potential, for example, may lead to future increases in births. Falls in fertility in populations where pregnancy, childbirth and early childhood are hazardous will lead to decreases in maternal and infant deaths. Migration affects other demographic parameters because migrants differ from the general population. International migrants who move voluntarily are generally young and in good health and often move from relatively high- to low-fertility populations. Consequently, immigrants may serve to (temporarily) 'rejuvenate' the host population and, at least initially, have higher fertility and lower mortality.
Changes in the size of a population produced by the surplus (or deficit) of births over deaths are termed natural increase (or decrease). If net migration is zero, this will be the same as the growth rate of the population – the overall annual change in the population divided by the population size. The number of births is a function of the number of potential mothers, to which immigration may contribute, as well as of their fertility patterns. The young age structures of many populations in the poorer regions of the world mean that these populations have a huge built-in potential for growth.
Demographic data and measures
Essentially all demographic analysis requires data both on the population 'stock' and on 'flows' in and out – births, deaths, and migration. The traditional sources of information on the former are population censuses and, for the latter, vital registration systems (routine recording of births and deaths). Although primarily a tool for collecting data on population 'stock', censuses are also used to find out about vital events. Many countries use censuses to provide data on migration (through questions on earlier place of residence or place of birth). Indirect estimation techniques developed by demographers mean that questions on number of children born and number who have died, on widowhood and orphanhood, are used to assess mortality levels and trends using both censuses and surveys in countries with deficient vital registration systems (see Tools for Demographic Estimation). Census taking is expensive and requires both an administrative infrastructure and the co-operation of the population to be enumerated. The 21st century is seeing more countries adopting alternatives as the information required by governments becomes more complex and the difficulties of mass data collection escalate.
Most high income countries have well-established vital registration systems with very near complete coverage. In poorer parts of the world vital registration systems are frequently seriously incomplete or non-existent and only about a third of deaths estimated to occur globally are registered and reported to the World Health Organisation, although if the sample registration systems in India and China are considered as sufficiently representative of their national populations, this proportion rises to close to three-quarters.
Many countries have a range of surveys which provide more detailed information on, for example, health-related behaviour or family building strategies that would be impossible to collect in a census. In poorer countries, where other data sources are scarcer, surveys often present the best source of data on basic demographic parameters. The World Fertility Survey (WFS), an international population research programme launched in 1972 to determine fertility levels throughout the world, and its successor, the Demographic and Health Survey Programme (DHS), have been valuable in providing demographic and health data for a range of poorer countries.
The raw materials of demography relate to individuals' most personal experiences – sexual activity, family formation, birth control, reproduction, marital breakdown, illness and death. All of these occur in a social framework which attaches value to some of these behaviours and stigmatises others. Not surprisingly, respondents in censuses and surveys may be reluctant to disclose non-marital pregnancies, illegal abortions or undocumented migration. Concealment has also been the policy of some national governments which have treated demographic data as official secrets. Additionally, people's uncertainties about age or other 'basic' characteristics, their uncertain recollections of prior events and the vast scope for administrative errors have to be considered. The demographer's traditional obsession with data quality is hence understandable.
The Analysis of Demographic Data
A standard array of techniques and measures forms the basis of much demographic analysis. Information on these measures, and how to use them, is now provided in the UNFPA/IUSSP on line training modules (as well as in many textbooks). Analysis involves not just the application of a particular technique, but decisions about what units of analysis to use and how to group them. A major distinction is between period and cohort analysis. Period analysis deals with events of a particular time period (for example, mortality rates 2005-2010) while cohort analyses follow the experience of individuals through time. Cohorts in this sense are defined as groups of people who have experienced the same significant event at the same time. Thus birth cohorts comprise people born in a particular year or group of years. Cohort and period are two of the dimensions which 'place' persons in time; the third is age. Duration effects (such as duration of marriage, proximity to death or length of exposure to a particular pathogen) may also be important.
Population projections represent one of the most widely used outputs of demographic analysis. Strictly speaking a projection simply represents the outcome of applying various assumptions about future fertility, mortality and migration and so differs from a forecast, which implies prediction. However, projections are often treated as forecasts and the degree of uncertainty inherent in them is not always sufficiently recognised, although the production of probabilistic forecasts, as in the latest UN projections, makes this more explicit. The most common method of projection is the component method. Assumptions are made about the three components of change – births, deaths and migration – and applied to age and sex groups within the initial population to give a projection of future size and structure. To a large extent assumptions are based on recent trends together with other information, for example, survey data on fertility intentions or (sometimes) models of change in particular causes of death.
Population – a paramount issue for the 21st century
Population size, growth and age structure are all outcomes of variations in demographic behaviours and all have enormous implications for population health and well-being. While some countries grapple with the needs of rapidly growing populations, such as large increases in requirements for child health services and schools and, in some, interactive effects of environmental degradation, others face challenges of population ageing and, in some cases, population decline. Migration, including the displacement of people due to conflict or persecution, presents challenges for both origin and destination countries. Issues such as climate change, economic and cultural globalisation, gender relations and social inequality all interact with demographic patterns and processes making the work of demographers and population scientists of increasing importance.
Emily Grundy 2014
Note: Part of this article draws on a chapter written by Emily Grundy and Michael Murphy for the Oxford Textbook of Global Public Health (6 ed.), edited by Roger Detels, Martin Gulliford, Quarraisha Abdool Karim, and Chorh Chuan Tan, Oxford University Press (2015) 3, 718-735.