Most population data come from population censuses, sample surveys, or vital registration records. They are collected by national governments, universities, and other research institutions.
Censuses attempt to survey the entire population.
Surveys obtain data on a representative sample of the total population and statistically estimate the population parameters from the sample data. Both official census data and sample survey data are used to project future population size and composition. Vital statistics are administrative records of births, deaths, and other demographic phenomena and are frequently the responsibility of local, rather than national, offices. Population censuses are conducted by most national governments at approximately ten year intervals for administrative and governmental purposes. For example, the United States Constitution requires the Bureau of the Census to conduct a population census every ten years to determine each state's representation in the U.S. House of Representatives.