Introduction
A population census is an official count of people and housing within a country or region at a specific point in time. Its primary goal is to create a comprehensive statistical snapshot of the nation and its inhabitants.
Nearly every country conducts a census regularly to gather vital information about the social and economic conditions of its population. These surveys are the most reliable source of detailed data for small geographic areas and specific population groups.
Because they are costly, labour-intensive, and time-consuming, censuses are typically conducted at regular but infrequent intervals – commonly every ten years, every five years, or occasionally at irregular intervals depending on the country.
In the years between censuses, population figures are estimated using vital statistics, provided these are sufficiently accurate. For example, the estimated population in a post-census year is calculated by taking the last census count, adding births, subtracting deaths, and adjusting for net migration during the intervening period.
Early censuses
Civilizations throughout history have recognised the importance of gathering information about their most valuable resource – their people. While the modern population census began to take shape in the 17th century, earlier societies conducted inventories of people or taxpayers for very different purposes.
The Babylonians and Chinese, for example, held censuses primarily for military conscription and taxation. In ancient Egypt, population data helped organise labour for pyramid construction and manage land redistribution after the Nile’s annual flooding. These early records were often inaccurate, as individuals had little incentive to report truthful information. Moreover, only select groups, such as heads of households, were typically counted, resulting in incomplete population data.
The Greeks and Romans also conducted censuses long before the Common Era. Roman authorities carried out a census every five years to assess citizens’ property and tax obligations. Initially limited to Rome, this practice expanded across the empire by 5 BC. One such census, ordered by Caesar Augustus, famously required all men to return to their ancestral towns – prompting Joseph and Mary’s journey to Bethlehem.
After the fall of the Roman Empire, census-taking largely disappeared in Western Europe until the 17th century. A notable exception was the Domesday Book, commissioned by William the Conqueror in 1086. This extensive survey of land and property in England was designed to inform the new king about his realm’s wealth and landowners. It was a monumental task for its time and took several years to complete.
Modern census
In modern times, the purpose of a population census has shifted from identifying or controlling individuals to providing a comprehensive enumeration of all people and their important characteristics. This data is essential for understanding societal structure and trends.
The methods used in census-taking have evolved gradually and continue to improve. Three major developments have shaped the modern census as practiced in most countries today:
- The widespread adoption of nationwide counts for governmental and commercial planning.
- Advances in enumeration techniques, along with legal safeguards to protect the confidentiality of individual responses.
- A significant increase in the depth, accuracy, and scope of the information collected.
One of the earliest censuses reflecting these modern principles was conducted in Quebec and Nova Scotia in 1665, followed by 16 more between then and 1754. Iceland implemented a census in 1703, Germany in 1742, and Sweden in 1749. Although several British North American colonies had already conducted full enumerations, the first official United States (U.S.) census was delayed until 1790 due to religious concerns. Many feared divine punishment, recalling the biblical account of King David’s census, which was followed by a devastating plague that killed 70,000 people.
The 1790 U.S. census was groundbreaking – not only for the vast area it covered and the demographic data it gathered, but also for its political significance. It was used to determine representation in Congress based on population, setting a precedent for the role of census data in democratic governance.
Census taking in Britain
Compared to other nations, Great Britain was slow to adopt census-taking. Like some in the early United States, many Britons believed that counting people was sacrilegious. This sentiment was a key argument against a 1753 bill introduced in the House of Commons proposing an annual census. According to MP Matthew Ridley, the people of Newcastle-upon-Tyne viewed the proposal as ominous, fearing that “an epidemical distemper should follow the numbering”.
However, most opposition stemmed from concerns that a census might reveal national weaknesses to foreign powers or infringe on individual liberties. The bill was ultimately defeated in the House of Lords.
By the late 18th century, it had become clear that no one truly knew how many people lived in the British Isles. Some believed the population was growing, while others feared it was in decline.
Resistance to an official census began to fade after the publication of Thomas Malthus’s influential Essay on the Principle of Population in 1798. Malthus warned that unchecked population growth could soon outpace food production and other resources, leading to famine, disease, and widespread hardship.
Alarmed by these predictions, public and political opinion shifted in favour of a national census. Parliament passed the Census Act in 1800, and the first official census of England and Wales was conducted on 10 March 1801.
Census taking in Ireland
While official census-taking arrived later in Ireland, earlier efforts – such as Sir William Petty’s Down Survey in 1672, which estimated the population at 1.1 million – provided valuable demographic and socio-economic insights.
The first official attempt at a modern census occurred in 1813. However, it was unsuccessful due to poor oversight: responsibility was delegated to the Grand Juries of various counties, which lacked the structure and capacity to carry out the task effectively. In some counties, no enumeration took place; in others, the results were unsatisfactory. The effort was abandoned after two years.
Subsequent censuses in 1821 and 1831 also faced challenges, but it was not until 1841 – when Ordnance Survey maps became available – that a comprehensive and reliable enumeration was achieved. From 1821 to 1911, censuses were conducted every ten years.
Political developments in Ireland led to the postponement of the 1921 census, breaking the synchronisation with the Great Britain Census schedule – a gap that was not restored until 1951. In Northern Ireland, the second reading of a Census Bill was introduced in 1925 by the Minister of Finance. He cited the need for updated data following the Great War and the 1918-19 influenza pandemic. The census was held on 18 April 1926, coinciding with a census in the rest of Ireland. Each head of household was required to complete a form for all residents – a method that remains in use today.
To bridge the gap between the 1926 and planned 1941 censuses, a limited census was conducted in 1937, omitting questions on occupation and industry. The outbreak of World War II and a resulting paper shortage led to a restricted publication of results and the eventual cancellation of the 1941 census.
Since 1951, censuses in Northern Ireland have aligned with those in the rest of the United Kingdom, occurring every ten years, apart from an additional midterm census in 1966.
Further reading: For a more detailed account, see Chapter 2 – A Brief History of the Census in Ireland/Northern Ireland (PDF, 4MB) from the Registrar General Northern Ireland Annual Report 2011, authored by Ian White of the Office for National Statistics.
Modern census procedures
The term census refers to the systematic enumeration of items important to a country – such as housing, agriculture, or manufacturing – at a specific point in time. However, when used without qualification, it typically refers to a census of population.
Modern censuses aim to count every individual within a clearly defined geographic area at a specific moment, known as the census moment. Two main approaches are used:
- De jure census: Counts individuals based on their usual or legal residence.
- De facto census: Counts individuals based on where they were physically present on the census day.
Both the definition of census geography and the timing of the census are critical. Detailed maps are essential for accurate geographic coverage, while the census moment serves as the reference point for time-sensitive data such as age or marital status.
A fixed questionnaire is used to collect direct data, including name, address, date of birth, and occupation. Some of this information guides the enumeration process, while most is used for analysis. The United Nations recommends the inclusion of the following topic areas:
- Geographic: Place of enumeration and/or usual residence.
- Familial: Relationship to the head of household or family.
- Demographic: Sex, age, marital status, children ever born, birthplace.
- Economic: Employment status, occupation, industry, employer-employee relationship.
- Social and Political: Citizenship, language, ethnic or religious affiliation.
- Educational: Literacy, educational attainment, school attendance.
From this direct data, statisticians derive additional variables – known as derived data – such as total population, household composition, and migration patterns. These are created by processing and combining responses to generate new indicators and classifications.
The analysis of census data has advanced significantly with the advent of computers. In the 19th century, all tabulations were done manually, limiting both the speed and complexity of analysis. Today, data can be processed rapidly and disseminated in a wide variety of formats and levels of detail.