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Multilingual Demographic Dictionary, second unified edition, English volume
72
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720
Population projections ^{1} are calculations which show the future development of a population when certain assumptions are made about the future course of population change, usually with respect to fertility, mortality and migration. They are in general purely formal calculations, developing the implications of the assumptions that are made. A population forecast ^{2} is a projection in which the assumptions are considered to yield a realistic picture of the probable future development of a population. Although the projection period ^{3} is variable, short-term forecasts ^{4} are the rule, as the margin of error to which forecasts are subject increases considerably as the length of the forecast’s period increases. The most frequently used method of projection is the component method ^{5} or cohort-component method ^{5} which takes the population distributed by age and sex at a base date ^{6} and carries it forward in time, cohort by cohort, on the basis of separate allowances for fertility, mortality and migration. When matrix algebra is used for component projections, the method is sometimes called matrix method of projection ^{7}.
- 1. Projections are also made in terms of educational, economic and social characteristics. Backward projections which might be more accurately called retrojections, use similar methods to trace the past evolution of the population.
721
Estimates of the population ^{1} by size and composition at various dates in the past and present may be made by various methods, including many of the methods used for population projections (720-1). Demographic estimates ^{2} include estimates of the population and of such characteristics as fertility (601-1), mortality (401-1), etc. The annual extrapolation of population ^{5★} from the last census is carried out on the basis of the last census and vital statistics in subsequent years. Intercensal estimates ^{3} relate to dates intermediate to two or more censuses (202-1), and take the results of these censuses into account. The error of closure ^{4} is the difference between the size of a population enumerated at a new census and the population estimated for that census date on the basis of a previous census, the excess of births over deaths, and net migration (805-2) during the intercensal period. This difference represents the balance of errors in the data on births, deaths, net migration, and the coverage of the two censuses.
- 3. Postcensal estimates take the results of a previous census into account, but not those of the next census.
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