A census is a survey conducted by a governing body of the entire population of a region. This region can be a country, state, city, or other geographical area, but is most commonly a national region. Censuses are primarily aimed at doing a count of the number of people who live in an area. However, censuses frequently ask for other information as well, such as how many persons live in each household, whether or not people are married, and the level of education for members in a household.

Ancient civilizations, such as the Babylonians, Romans, Egyptians, Chinese, and Greeks, all conducted basic censuses for taxation and military purposes. These surveys were incomplete by modern standards and often only counted landholding men and their property. Modern censuses and census methods began to emerge in the 17th century.

Censuses are performed at regular intervals by national governments. These intervals often range from five to 10 years—Canada conducts a census every five years, while the United States and Britain conduct a census every 10 years. This ensures that the government will be able to regularly update and compare information about the region’s population. Censuses are one way to determine precise information regarding the exact number of people living in a region, as opposed to using estimates. A census can be done in a variety of ways. Postal surveys and door-to-door interviews are the most common means of collecting census data. Census data are also used to create legislative districts. In the U.S., each congressional district is home to roughly the same number of people, as determined by the decennial census.

A census can be done for many reasons. The most obvious reason is to determine how many people are living in an area, and how this population group changes over time. This is crucial for determining government policy, such as representation in governing bodies. Information gathered during a census can also be used to predict the future movements of peoples by identifying certain population trends, such as worker migration. By asking the same questions across a nation, it becomes possible to compare regions to each other. These comparisons often impact the kinds of government and private programs established in different regions. Censuses also collect data about the economic status of the people living in an area, which can be used to direct the provision of services and identify other regional needs. For nongovernmental research purposes, censuses provide data from a region otherwise too large to survey. Censuses also gather statistical data on vulnerable populations, such as people with disabilities, children, seniors, and other groups. This information can help guide specific policies designed to support these vulnerable groups.