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.