Census

The term census usually refers to the complete process of preparation, collection, compilation, evaluation, analysis, and dissemination of data on demographic, social, economic, and housing characteristics. In most countries, population censuses provide the only complete set of statistical information for all residents. Such information is fundamental for any GIS developed for socioeconomic and, often, environmental analysis purposes.

While surveys collect more detailed information on topics of special interest, they tend to be based on fairly small samples that yield representative information only for large groups of the population. Censuses, on the other hand, collect a small set of essential indicators that are a critical component of the national statistical system. Statistical offices also conduct censuses of agriculture or of the economy (e.g., industrial establishments). In some instances, these are in fact large sample surveys that do not provide complete coverage. This entry focuses on population and housing censuses, which are usually carried out jointly.

The population and housing census is usually conducted by a national statistical agency. In the 2000 census round, censuses were conducted by 190 of the 233 countries and areas for which the United Nations Statistics Division tracks information. These censuses have four main characteristics:

• Universality: The census should cover the territory of the entire country, which ensures that all persons and all housing units are included. Sometimes, this goal is not achievable, for instance, if parts of a country experience conflict or if there are special population groups that are hard to enumerate, such as nomadic people. In those cases, a census may be complemented by statistical sampling.
• Individual enumeration: A census requires that each person or dwelling unit is enumerated individually so that their characteristics are captured separately. This facilitates cross-tabulation of census indicators and compilation of information for small areas.
• Simultaneity: A census is representative of the status at a specified time, usually a single day or a period of a few days in which the information is collected. Some questions, such as the birth or death of family members, refer to a longer reference period, such as the year before the census. A census is defined as de facto if it records persons according to their actual location at the time of enumeration or de jure if persons are referenced at their usual place of residence. This distinction can make a significant difference, for example, in countries with many temporary migrant workers.
• Defined periodicity: An important purpose of a census program is to provide information about changes over time. For instance, demographic projections tend to rely on information from two or more censuses. A census should therefore be conducted at regular intervals. Because of the large cost involved, most countries conduct censuses only every 10 years, although the periodicity is less regular in many developing countries.