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Abstract:
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The total fertility rate is an estimate of the number of children an average woman would have if current age-specific fertility rates remained constant during her reproductive years. The values are estimated using demographic models based on several kinds of demographic parameters: a country's population size, age and sex distribution, fertility and mortality rates by age and sex groups, growth rates of urban and rural populations, and the levels of internal and international migration. Information collected through recent population censuses and surveys is used to calculate or estimate these parameters, but accuracy varies. The U.N. Population Division compiles and evaluates census and survey results from all countries. These data are adjusted for overenumeration and underenumeration of certain age and sex groups (e.g., infants, female children, and young males), misreporting of age and sex distributions, and changes in definitions, when necessary. These adjustments incorporate data from civil registrations, population surveys, earlier censuses, and, when necessary, population models based on information from socioeconomically similar countries. (Because the figures have been adjusted, they are not strictly comparable to the official statistics compiled by the U.N. Statistical Office and published in the Demographic Yearbook.) After the figures for population size and age/sex composition have been adjusted, these data are scaled to 1990. Similar estimates are made for each 5-year period between 1950 and 1990. Historical data are used when deemed accurate, also with adjustments and scaling. However, accurate historical data do not exist for many developing countries. In such cases, the U.N. Population Division uses available information and demographic models to estimate the main demographic parameters. Projections are based on estimates of the 1990 base-year population. Age- and sex-specific mortality rates are applied to the base-year population to determine the number of survivors at the end of each 5-year period. Births are projected by applying age-specific fertility rates to the projected female population. Births are distributed by an assumed sex ratio, and the appropriate age- and sex-specific survival rates are applied. Future migration rates are also estimated on an age- and sex-specific basis. Combining future fertility, mortality, and migration rates yields the projected population size, average annual population change, and average annual increment to the population. Assumptions about future mortality, fertility, and migration rates are made on a country-by-country basis and, when possible, are based on historical trends. Four scenarios of population growth are created by using different assumptions about fertility (high fertility, medium fertility, low fertility, and constant fertility). For example, the medium-case scenario assumes medium levels of fertility---an assumption that may vary among countries. Refer to the original source for further details. Although projections may be of questionable quality, U.N. demographic models are based on surveys and censuses with well-understood qualities, which makes these data fairly reliable.
Data available on the CD-ROM World Population Prospect: The 2000 Revision prepared by the Population division, Dpartement of Economic and Social affairs.
http://www.un.org/esa/population/unpop.htm
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