A census of population is a count of people and housing in a country or area at a particular time. Its purpose is to provide a statistical portrait of the country and its population.
Almost every country in the world regularly carries out a census to collect important information about the social and economic situation of the people living in its various regions. A census is the only reliable source of detailed data on small areas or groups within the population.
Censuses, which are generally expensive, extremely labour intensive and time-consuming, are taken only at infrequent but usually regular intervals, every ten years in many countries, every 5 years or at irregular intervals in other countries.
In non-census years, the population is estimated with the aid of vital statistics, if these are sufficiently reliable. For instance, the population in a post-censal year equals the population at the last census plus the births, minus the deaths, plus or minus the net migration during the intervening years.
Civilisations of every era have recognised the need to collect information on their most valuable asset - their people. While the modern population census began to evolve only in the 17th century before that time inventories of people or taxpayers were made. The methods and purposes of these inventories were very different from modern ones.
The Babylonians and the Chinese held censuses mainly for military and taxation purposes (i.e. identifying who should be inducted into military service and those who should be taxed). The Egyptians collected information on the population so that they could plan armies of people to build their giant pyramids and to redistribute land following the annual flooding of the Nile. These inventories were highly inaccurate as it was generally not in an individual's best interest to provide correct information. Additionally only people falling into particular categories such as family heads were counted, thereby resulting in a non-representative sample of the population.
The Greeks and Romans held censuses of population many years before the birth of Christ. The Romans enumerated their citizens and property every five years to determine their liabilities. Initially, this was conducted only in Rome but was extended to the entire Roman Empire in 5 BC. It was the five-yearly census ordered by Caesar Augustus which required every man in the Roman Empire to return to his place of origin, thus ensuring that Joseph and Mary travelled to Bethlehem for the birth of Jesus.
Upon the collapse of the Roman Empire, the practice was discontinued in the West until the 17th century with the exception of the completion of the Domesday Book. This detailed inventory of land and property was completed in 1086 in the British Isles when William the Conqueror ordered its production as a method for acquainting him with the landowners and holdings of his new domain. It was a massive undertaking at the time and took several years to complete.
In more modern times the need for a population census has moved away from a requirement to identify and even control particular individuals to become a complete enumeration of all people and their important characteristics for information regarding the structure and trends in society.
The mechanisms used in modern Census taking have also developed slowly and are still constantly evolving. Three parallel developments are accredited with the current census procedures that are commonly used by the majority of the world's countries:
- the gradual spread of country wide counts for governmental and business purposes
- the improvement of the techniques used in enumeration and the introduction of legal safeguards to ensure the confidentiality of an individual responses
- an increase in the depth and accuracy of the information obtained
The first Census embodying some of these modern principles occurred in Quebec and Nova Scotia in 1665, with a further 16 taking place between then and 1754. Iceland followed in 1703, Germany in 1742 and Sweden in 1749. While several British North American colonies had made full enumerations, the first United States census was delayed until 1790 because of religious opposition. People feared that a census might incur the wrath of God because a census of the Israelites ordered by King David was followed by a plague which killed 70,000 people. This census made history in both the size of the area enumerated and the effort to obtain data on characteristics of the population but also because of the political purpose for which it was undertaken - namely, representation in Congress on the basis of population.
Census taking in Britain
Great Britain was slow to follow suit. In Britain, some believed, like those in the United States, that any type of people count was sacrilegious. This view was used as an argument against census taking in Britain when a Bill was introduced into the House of Commons in 1753 to carry out an annual Census. The people of Newcastle-upon-Tyne ‘looked upon the proposal as ominous and feared… an epidemical distemper should follow the numbering’, according to Matthew Ridley, their MP. Most opposition, however, was concerned with a fear that the results would disclose to foreign enemies the weakness of the country or that the exercise would impair individual liberty and it was defeated in the Lords.
Towards the end of the 18th Century, however, it became increasingly obvious that nobody had any idea about the number of people living in the British Isles. Some said the population of Britain was rising while others were sure it was falling.
Opposition to an official census finally withered away after the famous demographer Thomas Malthus published his essay on the ´principle of population´ in 1798. Malthus caused great concern by suggesting that population growth would soon outstrip supplies of food and other resources. Unable to support itself, Britain would be hit by famine, disease and other disasters.
Frightened by this alarmist view of the future, people began to see the need for a census. Parliament passed the Census Act in 1800 and the first official Census in England and Wales was held on 10 March 1801.
Census taking in Ireland
Official census-taking came later to Ireland although social historians rightly point to the rich source of demographic and socio-economic data available from earlier work such as that of Sir William Petty in the Down Survey. (Petty had estimated the population of Ireland at 1.1 million in 1672). However, the first official attempt at modern enumeration came in 1813. Sadly, it was not a success, the supervision of the enumeration having been passed to the Grand Juries of the various counties who were not adequately structured to accomplish the task. In some counties, no steps were taken whilst in others the work completed was unsatisfactory. After two years the attempt was abandoned. Both the censuses of 1821 and 1831 also experienced certain difficulties - and it was not until 1841 when Ordnance Survey Maps were available that a total comprehensive enumeration was achieved.
From 1821 censuses were taken at ten yearly intervals until 1911. Subsequently, events in Ireland resulted in the postponement of the 1921 Census. This resulted in a break with Great Britain Census timing - one which was not restored until 1951. It was not until 1925 that the Northern Ireland Minister of Finance introduced the second reading on a Census Bill. He indicated that it was nearly 15 years since the last Census in Ireland and that intervening events such as the Great War and the 1918-19 flu epidemic required a census to be carried out. It was accordingly held on 18 April 1926 (the day also of a census in the rest of Ireland). The head of each household was given a form to fill in on behalf of everyone in the dwelling on a certain day. This system has stood the test of time and it still forms the basis of the method we use today.
In order to bridge the gap between the census of 1926 and the planned census of 1941 a census of more limited scope (e.g. omitting questions regarding occupation and industry) was taken in 1937. The outbreak of war and the subsequent paper shortage led to a restricted publication programme and later, inevitably, to the abandonment of plans for the 1941 census. Since 1951 Censuses in Northern Ireland have been held at the same time as the rest of the United Kingdom at ten yearly intervals, with the exception of the additional 1966 midterm census.
Chapter 2 - A Brief History of the Census in Ireland/Northern Ireland (PDF 4MB) from the Registrar General Northern Ireland Annual Report 2011 presents a look at the history of the census throughout Ireland before 1921 and in Northern Ireland since 1926. This chapter was written by Ian White from the Office for National Statistics.
Modern census procedures
The term census refers to any count/enumeration of items that are important to a country, such as housing, agriculture or manufacturing, taken at a particular time. Used on its own, however, it usually refers to a census of population.
Modern censuses aim to enumerate every person within a precisely delimited territory and sub-area at a particular moment in time (known as the census moment) using one of two approaches:
- A de jure census which tallies people according to their regular or legal residence
- A de facto census which allocates them to the place where they spent the night on the day of enumeration
The definition of the census geography and the census moment are crucial in the planning and conduct of a census with the former requiring the use of extremely detailed maps and the later providing the chronological line separating the included from the excluded as well as the reference point for questions such as age and marital status.
A fixed questionnaire is used for interviewing and provides direct data such as name, address, date of birth and occupation, some of which is used solely to guide the enumeration process while most is used in the analysis stages. The following is one list of desirable topics suggested by the United Nations:
- Geographic - Place enumerated and/or place of usual residence
- Familial - Relation to head of household or family
- Demographic - Sex, age, marital status, children ever born, birthplace
- Economic - Type of activity, occupation, industry, employer-employee status
- Social and Political - Citizenship, language, ethnic or religious affiliation
- Educational - literacy or level of education, school attendance
The direct data can in turn be used to derive an entire set of new variables known as derived data. This involves performing operations on one or more of the direct data variables to create new values, classifications or indicators such as total population, family composition and migration data.
The depth and speed of analysis of census data has evolved considerably with the introduction of computers. In the 19th century, tabulations were completed manually thus restricting the amount and complexity of the analysis performed. Today tabulations can be made relatively quickly and products are made available in a wide range of formats and detail.