SamplingA census is often construed as the opposite of a sample as its intent is to count everyone in a population rather than a fraction. However, population censuses do rely on a to count the population. This is the only way to be sure that everyone has been included as otherwise those not responding would not be followed up on and individuals could be missed. The fundamental premise of a census is that the population is not known and a new estimate is to be made by the analysis of primary data. The use of a sampling frame is counterintuitive as it suggests that the population size is already known. However, a census is also used to collect attribute data on the individuals in the nation, not only to assess population size. This process of sampling marks the difference between a historical census, which was a house to house process or the product of an imperial decree, and the modern statistical project. The sampling frame used by census is almost always an address register. Thus it is not known if there is anyone resident or how many people there are in each household. Depending on the mode of enumeration, a form is sent to the householder, an enumerator calls, or administrative records for the dwelling are accessed. As a preliminary to the dispatch of forms, census workers will check any address problems on the ground. While it may seem straightforward to use the postal service file for this purpose, this can be out of date and some dwellings may contain a number of independent households. A particular problem is what are termed 'communal establishments' which category includes student residences, religious orders, homes for the elderly, people in prisons etc. As these are not easily enumerated by a single householder, they are often treated differently and visited by special teams of census workers to ensure they are classified appropriately.
Residence definitionsIndividuals are normally counted within households, and information is typically collected about the household structure and the housing. For this reason international documents refer to censuses of population and housing. Normally the census response is made by a household, indicating details of individuals resident there. An important aspect of census enumerations is determining which individuals can be counted and which cannot be counted. Broadly, three definitions can be used: ''de facto'' residence; ''de jure'' residence; and permanent residence. This is important in considering individuals who have multiple or temporary addresses. Every person should be identified uniquely as resident in one place; but the place where they happen to be on , their ''de facto'' residence, may not be the best place to count them. Where an individual uses services may be more useful, and this is at their usual residence. An individual may be recorded at a "permanent" address, which might be a family home for students or long term migrants. A precise definition of residence is needed, to decide whether visitors to a country should be included in the population count. This is becoming more important as students travel abroad for education for a period of several years. Other groups causing problems of enumeration are new-born babies, refugees, people away on holiday, people moving home around census day, and people without a fixed address. People with second homes because they are working in another part of the country or have a holiday cottage are difficult to fix at a particular address; this sometimes causes double counting or houses being mistakenly identified as vacant. Another problem is where people use a different address at different times e.g. students living at their place of education in term time but returning to a family home during vacations, or children whose parents have separated who effectively have two family homes. Census enumeration has always been based on finding people where they live, as there is no systematic alternative: any list used to find people is likely to be derived from census activities in the first place. Recent UN guidelines provide recommendations on enumerating such complex households. In the census of agriculture, data is collected at the agricultural holding unit. An agricultural holding is an economic unit of agricultural production under single management comprising all livestock kept and all land used wholly or partly for agricultural production purposes, without regard to title, legal form, or size. Single management may be exercised by an individual or household, jointly by two or more individuals or households, by a clan or tribe, or by a juridical person such as a corporation, cooperative or government agency. The holding's land may consist of one or more parcels, located in one or more separate areas or in one or more territorial or administrative divisions, providing the parcels share the same production means, such as labour, farm buildings, machinery or draught animals.
Enumeration strategiesHistorical censuses used crude enumeration assuming absolute accuracy. Modern approaches take into account the problems of overcount and undercount, and the coherence of census enumerations with other official sources of data. This reflects a realist approach to measurement, acknowledging that under any definition of residence there is a true value of the population but this can never be measured with complete accuracy. An important aspect of the census process is to evaluate the quality of the data. Many countries use a post-enumeration survey to adjust the raw census counts. This works in a similar manner to estimation for animal populations. Among census experts this method is called dual system enumeration (DSE). A sample of households are visited by interviewers who record the details of the household as at census day. These data are then matched to census records, and the number of people missed can be estimated by considering the numbers of people who are included in one count but not the other. This allows adjustments to the count for non-response, varying between different groups. An explanation using a fishing analogy can be found in "Trout, Catfish and Roach..." which won an award from the for excellence in in 2011. Triple system enumeration has been proposed as an improvement as it would allow evaluation of the statistical dependence of pairs of sources. However, as the matching process is the most difficult aspect of census estimation this has never been implemented for a national enumeration. It would also be difficult to identify three different sources that were sufficiently different to make the triple system effort worthwhile. The DSE approach has another weakness in that it assumes there is no person counted twice (over count). In ''de facto'' residence definitions this would not be a problem but in ''de jure'' definitions individuals risk being recorded on more than one form leading to double counting. A particular problem here is students who often have a term time and family address. Several countries have used a system which is known as short form/long form. This is a strategy which randomly chooses a proportion of people to send a more detailed questionnaire to (the long form). Everyone receives the short form questions. This means more data are collected, but without imposing a burden on the whole population. This also reduces the burden on the statistical office. Indeed, in the UK until 2001 all residents were required to fill in the whole form but only a 10% sample were coded and analysed in detail. New technology means that all data are now scanned and processed. During the there was controversy about the cessation of the mandatory long form census; the head of , , resigned upon the federal government's decision to do so. The use of alternative enumeration strategies is increasing but these are not as simple as many people assume, and are only used in developed countries. The Netherlands has been most advanced in adopting a census using . This allows a simulated census to be conducted by linking several different administrative databases at an agreed time. Data can be matched and an overall enumeration established allowing for discrepancies between different data sources. A validation survey is still conducted in a similar way to the post enumeration survey employed in a traditional census. Other countries which have a population register use this as a basis for all the census statistics needed by users. This is most common among Nordic countries, but requires many distinct registers to be combined, including population, housing, employment and education. These registers are then combined and brought up to the standard of a statistical register by comparing the data in different sources and ensuring the quality is sufficient for official statistics to be produced. A recent innovation is the French instigation of a rolling census programme with different regions enumerated each year, so that the whole country is completely enumerated every 5 to 10 years. In Europe, in connection with the 2010 census round, many countries adopted alternative census methodologies, often based on the combination of data from registers, surveys and other sources.
TechnologyCensuses have evolved in their use of technology: censuses in 2010 used many new types of computing. In Brazil, handheld devices were used by enumerators to locate residences on the ground. In many countries, census returns could be made via the Internet as well as in paper form. DSE is facilitated by computer matching techniques which can be automated, such as . In the UK, all census formats are scanned and stored electronically before being destroyed, replacing the need for physical archives. The record linking to perform an administrative census would not be possible without large databases being stored on computer systems. There are sometimes problems in introducing new technology. The US census had been intended to use handheld computers, but cost escalated and this was abandoned, with the contract being sold to Brazil. Online response has some advantages, but one of the functions of the census is to make sure everyone is counted accurately. A system which allowed people to enter their address without verification would be open to abuse. Therefore, households have to be verified on the ground, typically by an enumerator visit or post out. Paper forms are still necessary for those without access to the internet. It is also possible that the hidden nature of an administrative census means that users are not engaged with the importance of contributing their data to official statistics. Alternatively, population estimations may be carried out remotely with and technologies.
DevelopmentAccording to UNFPA, "The information generated by a population and housing census – numbers of people, their distribution, their living conditions and other key data – is critical for development." This is because this type of data is essential for policymakers so that they know where to invest. Unfortunately, many countries have outdated or inaccurate data about their populations and thus have difficulty in addressing the needs of the population. UNFPA said:
"The unique advantage of the census is that it represents the entire statistical universe, down to the smallest geographical units, of a country or region. Planners need this information for all kinds of development work, including: assessing demographic trends; analysing socio-economic conditions; designing evidence-based poverty-reduction strategies; monitoring and evaluating the effectiveness of policies; and tracking progress toward national and internationally agreed development goals."In addition to making policymakers aware of population issues, the census is also an important tool for identifying forms of social, demographic or economic exclusions, such as inequalities relating to race, ethics, and religion as well as disadvantaged groups such as those with disabilities and the poor. An accurate census can empower local communities by providing them with the necessary information to participate in local decision-making and ensuring they are represented. The importance of the census of agriculture for development is that it gives a snapshot of the structure of the agricultural sector in a country and, when compared with previous censuses, provides an opportunity to identify trends and structural transformations of the sector, and points towards areas for policy intervention. Census data are used as a benchmark for current statistics and their value is increased when they are employed together with other data sources.
Uses of census dataEarly censuses in the 19th century collected paper documents which had to be collated by hand, so the statistical information obtained was quite basic. The government owned the data could publish statistics on the state of the nation. The results were used to measure changes in the population and apportion representation. Population estimates could be compared to those of other countries. By the beginning of the 20th century, censuses were recording households and some indications of their employment. In some countries, census archives are released for public examination after many decades, allowing genealogists to track the ancestry of interested people. Archives provide a substantial historical record which may challenge established views. Information such as job titles and arrangements for the destitute and sick may also shed light on the historical structure of society. Political considerations influence the census in many countries. In Canada in 2010 for example, the government under the leadership of Stephen Harper abolished the mandatory long-form census. This abolition was a response to protests from some Canadians who resented the personal questions. The long-form census was reinstated by the Justin Trudeau government in 2016.
Census data and researchAs governments assumed responsibility for schooling and welfare, large government departments made extensive use of census data. Population projections could be made, to help plan for provision in local government and regions. Central government could also use census data to allocate funding. Even in the mid 20th century, census data was only directly accessible to large government departments. However, computers meant that tabulations could be used directly by university ers, large businesses and local government offices. They could use the detail of the data to answer new questions and add to local and specialist knowledge. Nowadays, census data are published in a wide variety of formats to be accessible to business, all levels of government, media, students and teachers, charities, and any citizen who is interested; researchers in particular have an interest in the role of Census Field Officers (CFO) and their assistants. Data can be represented visually or analysed in complex statistical models, to show the difference between certain areas, or to understand the association between different personal characteristics. Census data offer a unique insight into small areas and small demographic groups which sample data would be unable to capture with precision. In the census of agriculture, users need census data to: # support and contribute to evidence-based agricultural planning and policy-making. The census information is essential, for example, to monitor the performance of a policy or programme designed for crop diversification or to address food security issues; # provide data to facilitate research, investment and business decisions both in the public and private sector; # contribute to monitoring environmental changes and evaluating the impact of agricultural practices on the environment such as tillage practices, crop rotation or sources of greenhouse gas (GHG) emissions; # provide relevant data on work inputs and main work activities, as well as on the labour force in the agriculture sector; # provide an important information base for monitoring some key indicators of the Sustainable Development Goals (SDGs), in particular those goals related to food security in agricultural holdings, the role of women in agricultural activities and rural poverty; # provide baseline data both at the national and small administrative and geographical levels for formulating, monitoring and evaluating programmes and projects interventions; # provide essential information on and for the estimation of the non-observed economy, which plays an important role in the compilation of the national accounts and the economic accounts for agriculture.
Privacy and data stewardshipAlthough the census provides useful statistical information about a population, the availability of this information could sometimes lead to abuses, political or otherwise, by the linking of individuals' identities to anonymous census data. This 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 is done to reduce the privacy risk, new improved electronic analysis of data can threaten to reveal sensitive individual information. This is known as statistical disclosure control. 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 ( s) can be derived interactively from the estimated 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. Different release strategies between government have led to an international project ( ) to co-ordinate access to microdata and corresponding metadata. Such projects such as SDMX also promote standardising metadata, so that best use can be made of the minimal data available.
History of censuses
EgyptCensuses in Egypt first appeared in the late Middle Kingdom and developed in the Pharaoh Amasis, according to , required every Egyptian to declare annually to the , "whence he gained his living". Under the Ptolemies and the several censuses were conducted in Egypt by government officials
Ancient GreeceThere are several accounts of ancient Greek city states carrying out censuses.
IsraelCensuses are mentioned in the . God commands a per capita tax to be paid with the census for the upkeep of the . The is named after the counting of the Israelite population according to the house of the Fathers after the exodus from Egypt. A second census was taken while the Israelites were camped in the plains of . King performed a census that produced disastrous results. His son, King , had all of the foreigners in Israel counted. When the Romans took over Judea in AD6, the legate organised a for tax purposes. The links the birth of Jesus either to this event, or to an otherwise unknown census conducted prior to Quirinius’ tenure.
ChinaOne of the world's earliest preserved censuses was held in China in AD2 during the , and is still considered by scholars to be quite accurate.Twitchett, D., Loewe, M., and Fairbank, J.K. ''Cambridge History of China: The Ch'in and Han Empires 221 B.C.–A.D. 220''. Cambridge University Press (1986), p. 240. The population was registered as having 57,671,400 individuals in 12,366,470 households but on this occasion only taxable families had been taken into account - indicating the income and the number of soldiers who could be mobilized. Another census was held in AD144.
IndiaThe oldest recorded census in India is thought to have occurred around 330BC during the reign of Emperor under the leadership of or Chanakya and .
RomeThe English term is taken directly from the ''census'', from ' ("to estimate"). The census played a crucial role in the administration of the Roman government, as it was used to determine the class a citizen belonged to for both military and tax purposes. Beginning in the middle republic, it was usually carried out every five years. It provided a register of citizens and their property from which their duties and privileges could be listed. It is said to have been instituted by the Roman king in the at which time the number of arms-bearing citizens was supposedly counted at around 80,000. The 6AD " " undertaken following the imposition of direct Roman rule in was partially responsible for the development of the movement and several failed rebellions against Rome that ended in the . The 15-year cycle established by in AD297 was based on quindecennial censuses and formed the basis for dating in late antiquity and under the .
Rashidun and Umayyad CaliphatesIn the , the began conducting regular censuses soon after its formation, beginning with the one ordered by the second , .
Medieval EuropeThe was undertaken in AD1086 by so that he could properly tax the land he had recently conquered. In 1183, a census was taken of the r , to ascertain the number of men and amount of money that could possibly be raised against an invasion by , sultan of and . 1328 : First national census of France (''L'État des paroisses et des feux'') mostly for fiscal purposes. It estimated the French population at 16 to 17 millions.
Inca EmpireIn the 15th century, the had a unique way to record census information. The Incas did not have any written language but recorded information collected during censuses and other numeric information as well as non-numeric data on , strings from or hair or cotton cords with numeric and other values encoded by knots in a positional system.
Spanish EmpireOn May 25, 1577, ordered by royal cédula the preparation of a general description of Spain's holdings in the Indies. Instructions and a questionnaire, issued in 1577 by the Office of the Cronista Mayor, were distributed to local officials in the Viceroyalties of New Spain and Peru to direct the gathering of information. The questionnaire, composed of fifty items, was designed to elicit basic information about the nature of the land and the life of its peoples. The replies, known as "", were written between 1579 and 1585 and were returned to the Cronista Mayor in Spain by the Council of the Indies.
World population estimatesThe earliest estimate of the world population was made by in 1661; the next by Johann Peter Süssmilch in 1741, revised in 1762; the third by in 1859. In 1931, Walter Willcox published a table in his book, ''International Migrations: Volume II Interpretations'', that estimated the 1929 world population to be roughly 1.8 billion.
Impact of COVID-19 on census
Impactpredicts that the will threaten the successful conduct of censuses of population and housing in many countries through delays, interruptions that compromise quality, or complete cancellation of census projects. Domestic and donor financing for census may be diverted to address COVID-19 leaving census without crucial funds. Several countries have already taken decisions to postpone the census, with many others yet to announce the way forward. In some countries this is already happening. The pandemic has also affected the planning and implementation of censuses of agriculture in all world's regions. The extent of the impact has varied according to the stages at which the censuses are, ranging from planning (i.e. staffing, procurement, preparation of frames, questionnaires), fieldwork (field training and enumeration) or data processing/analysis stages. The census of agriculture's reference period is the agricultural year. Thus, a delay in any census activity may be critical and can result in a full year postponement of the enumeration if the agricultural season is missed. Some publications have discussed the impact of COVID-19 on national censuses of agriculture.
AdaptationUNFPA has requested a global effort to assure that even where census is delayed, census planning and preparations are not cancelled, but continue in order to assure that implementation can proceed safely when the pandemic is under control. While new census methods, including online, register-based, and hybrid approaches are being used across the world, these demand extensive planning and preconditions that cannot be created at short notice. The continuing low supply of to protect against COVID-19 has immediate implications for conducting census in communities at risk of transmission. UNFPA Procurement Office is partnering with other agencies to explore new supply chains and resources.
See also* * * * *
References* Alterman, Hyman, (1969). ''Counting People: The Census in History''. Harcourt, Brace & Company. * Behrisch, Lars. (2016) "Statistics and Politics in the 18th Century." ''Historical Social Research/Historische Sozialforschung'' (2016): 238–57. * Bielenstein, Hans, (1978). "Wang Mang, the restoration of the Han dynasty, and Later Han." In ''The Cambridge History of China'', vol. 1, eds. Denis Twitchett and John K. Fairbank, pp. 223–90, Cambridge: Cambridge University Press. * Krüger, Stephen, (Fall 1991). "The Decennial Census"