A census is the procedure of systematically acquiring and recording information about the members of a given population. It is a regularly occurring and official count of a particular population. The term is used mostly in connection with national population and housing censuses; other common censuses include agriculture, business, and traffic censuses. In the latter cases the elements of the 'population' are farms, businesses, and so forth, rather than people. The United Nations defines the essential features of population and housing censuses as "individual enumeration, universality within a defined territory, simultaneity and defined periodicity", and recommends that population censuses be taken at least every 10 years. The term itself comes from Latin: during the Roman Republic the census was a list that kept track of all adult males fit for military service.
The census can be contrasted with sampling in which information is obtained only from a subset of a population, sometimes as an Intercensal estimate. Census data is commonly used for research, business marketing, and planning, as well as a baseline for sampling surveys. In some countries, census data are used to apportion electoral representation (sometimes controversially - e.g., Utah v. Evans).
Although the census provides a useful way of obtaining statistical information about a population, such information can sometimes lead to abuses, political or otherwise, made possible by the linking of individuals' identities to anonymous census data. This consideration is particularly important when individuals' census responses are made available in microdata form, but even aggregate-level data can result in privacy breaches when dealing with small areas and/or rare subpopulations.
For instance, when reporting data from a large city, it might be appropriate to give the average income for black males aged between 50 and 60. However, doing this for a town that only has two black males in this age group would be a breach of privacy because either of those persons, knowing his own income and the reported average, could determine the other man's income.
Typically, census data are processed to obscure such individual information. Some agencies do this by intentionally introducing small statistical errors to prevent the identification of individuals in marginal populations; others swap variables for similar respondents. Whatever measures have been taken to reduce the privacy risk in census data, new technology in the form of better electronic analysis of data poses increasing challenges to the protection of sensitive individual information.
Another possibility is to present survey results by means of statistical models in the form of a multivariate distribution mixture . The statistical information in the form of conditional distributions (histograms) can be derived interactively from the estimated mixture model without any further access to the original database. As the final product does not contain any protected microdata, the model based interactive software can be distributed without any confidentiality concerns.
Another method is simply to release no data at all, except very large scale data directly to the central government.
The Statistical Department of the Ministry of Finance conducted the first census in 1882, which considered as a preparatory step; the first true population census was conducted in 1897. Thereafter, censuses were conducted at ten-year intervals in 1907, 1917, 1927 and so on.
Censuses are mentioned in the Bible. God commands a flat tax to be paid with the census in Exodus 30:11-16 ! for the upkeep of the Tabernacle. The Book of Numbers is named after the counting of the Israelite population (in Numbers 1-4 !) according to clans after the exodus from Egypt. A second census was taken while the Israelite were camped in the plains of Moab, in Numbers 26 !.