Modern national governments and international organizations place a high priority on the accurate determination of national and worldwide populations. Describing the present population and predicting those of the future with reasonable accuracy requires reliable data. c6y17yj
Methods of Research
National censuses, civil registration, and, since the 1960s, national sample surveys are the major sources of demographic data. They provide the raw materials for investigating the causes and consequences of population changes. The most common source is the population census, a count of all persons by age and with specified social and economic characteristics within a given area at a particular time. A register is a continuous record of births, deaths, migrations, marriages, and divorces, often maintained by a local government; reliability varies with the scrupulousness of citizens in reporting these data. In the sample survey, a statistically selected portion is used to represent the total population.
In the U.S., decennial censuses have been taken since 1790. Since the 1950s the U.S. Bureau of the Census has conducted an annual Current Population Survey, a highly detailed sample survey of many aspects of demographic behavior and related socioeconomic factors. International population data are compiled in systematic form by the United Nations Statistical Office, which prepares an annual Demographic Yearbook; by the United Nations Demographic Division, which issues biennial assessments and projections of world population; and by the International Bank for Reconstruction and Development.

Measures of Population

The numbers of births, deaths, immigrants, and emigrants over a specified time interval determine the change in population size. For comparative purposes, these components of change are expressed as proportions of the total population, to yield the birth rate, death rate, migration rates, and the population growth rate. (Birth and death rates typically are stated as numbers per 1000 population per year.) These rates are affected by the age-composition of the population; for example, a very healthy population, which, as a result, has a relatively large proportion of old people, might have a death rate similar to that of a poor population made up of predominantly younger members. Demographers, therefore, often use measures that are free of this age-distribution influence. Two such widely used measures are the total fertility rate (TFR) and the life expectancy at birth.
The total fertility rate is the number of children a woman would have during her reproductive life if she experienced the prevailing rates of fertility at each age. High-fertility countries may have birth rates of 40 or even 50 per 1000 population (per year); corresponding levels of the TFR would be 5 to 7 children per woman. Low-fertility countries have birth rates of 15 to 20 per 1000 and TFRs of about 2. “Replacement level” fertility (the level at which each person on average has a single successor in the next generation) corresponds to a TFR of about 2.1 under low-mortality conditions.
The life expectancy at birth is the average length of life that would be observed in a population in which the currently prevailing mortality risks at each age continued indefinitely. Preindustrial populations were characterized by large fluctuations in mortality; long-run averages, however, would probably have shown death rates of 30 to 40 per 1000 and life expectancies of 25 to 35 years. Under modern health conditions, death rates below 10 per 1000 and life expectancies above 70 years are common.
Another important mortality measure is the infant mortality rate. This is the probability of death in the first year of life, usually stated as a number per 1000 births. Many less-developed countries have infant mortality rates above 100 per 1000—that is, more than 10 percent of the children die in their first year. In countries with effective health and educational systems, infant mortality rates are about 15 per 1000, or even lower.