Census

A census is a way of gathering and organizing information about a group of people or things. The word comes from Latin and originally meant counting all adult males who could serve in the military in ancient Rome. Today, a census is used to collect details about entire populations, like how many people live in a country and their characteristics.

The United Nations says a good census should count everyone in a specific area at the same time, and it should happen regularly, usually every ten years. This helps countries compare their data fairly with one another. Censuses can collect many types of information, not just the number of people. They might look at things like age, education, or where people live.

Censuses are important for many reasons. They help governments plan services, businesses understand their customers, and researchers study populations. The data from a census can also decide how resources or political representation are shared among different regions. While samples can sometimes give good estimates, a full census provides the most complete picture of a population.

History

Iran

One of the earliest systematic censuses happened long ago during the early Achaemenid period, up until the time of Darius The Great in Ancient Iran. This census helped with financial planning, military organization, and tax collection. It covered areas in Asia, Africa, and Europe. The census collected information about how many people lived in different places, how much money cities and areas had, and details about farming lands and resources.

Egypt

The first Egyptian census counted animals, especially cows, to help with taxes. Early on, this happened every two years, but later it became more frequent. Later, censuses in Egypt also started to count people. During the time of the Ptolemies and the Romans, government workers carried out several censuses in Egypt.

Ancient Greece

There are stories about ancient Greek cities doing censuses too.

Israel

The Bible talks about several censuses. For example, God asked for a special payment linked to a census to help maintain a special place called the Tabernacle. The Book of Numbers gets its name from counting the Israelite people after they left Egypt. There was another census later when the Israelites were staying in the "plains of Moab". King David did a census. His son, King Solomon, counted people from other places living in Israel.

China

One of the earliest known censuses was done in China in the year AD 2 during the Han dynasty. Scholars think it was very accurate. It recorded 57,671,400 people in 12,366,470 families. Another census happened in AD 144.

India

The oldest known census in India might have been around 330 BC during the time of Emperor Chandragupta Maurya.

Rome

See also: Roman censor and Indiction

The word "census" comes from Latin, meaning "to estimate." In ancient Rome, censuses were very important for deciding citizens' duties and rights for the military and taxes. They usually happened every five years starting from the middle of the Roman Republic. It is believed to have started with the Roman king Servius Tullius in the 6th century BC.

In AD 6, a Roman leader named Publius Sulpicius Quirinius organized a census in a place called Judaea to collect taxes. This event is mentioned in the Gospel of Luke in connection with the birth of Jesus.

Rashidun and Umayyad Caliphates

During the Middle Ages, leaders called Caliphs started doing regular censuses not long after they began their rule. This started with a census ordered by the second Rashidun caliph, Umar.

Medieval Europe

In AD 1086, William I of England did a big census called the Domesday Book to help him tax the land he had taken over. In 1183, a census was done in the crusader Kingdom of Jerusalem to find out how many people and how much money could be gathered to defend against attacks.

The first national census of France was done in 1328, mostly to help with taxes. It estimated that about 16 to 17 million people lived there.

Inca Empire

In the 15th century, the Inca Empire had a special way to keep track of information during censuses. They didn’t write things down, but instead used objects called quipus. These were strings made from llama or alpaca hair or cotton, with knots that represented numbers and other information.

Spanish Empire

On May 25, 1577, King Philip II of Spain asked for a big project to describe Spain’s lands in the Americas. Local leaders were given questions to ask people about their lives and land. The answers, called "relaciones geográficas", were collected between 1579 and 1585 and sent back to Spain.

Sampling

A census tries to count every person in a group, unlike a sample which only looks at some people. But even censuses use something called a sampling frame to make sure no one is missed. This can seem odd because it looks like we already know how many people there are, but a census also collects information about people, not just counts them.

Usually, the sampling frame is a list of addresses. We don’t always know how many people live at each address. Depending on how the census is done, forms might be sent to addresses, a worker might visit, or records might be checked. Before sending forms, workers check the addresses to make sure they are correct. Using postal files can be tricky because they might be old, and some places might have many households. Special places like student homes, religious groups, or homes for older people need special attention to count everyone correctly.

Residence definitions

People are usually counted where they live, called households. We collect information about these homes and the people living there. It is important to decide who to count and where. There are three main ways to decide this: where someone is staying on the day of the census (de facto residence); where they usually live (de jure residence); and their permanent home. This helps make sure everyone is counted once and only once, even if they have more than one address.

Some groups can be hard to count, like students studying in another country, babies who are very new, or people on holiday. People with second homes can also make counting tricky. The UN gives advice on how to count these groups properly.

In the census of agriculture, we collect data from farms. A farm is a place where farming happens under one management, whether it is run by one person, a family, or even a company. The farm can have many pieces of land in different places, but they all work together as one unit.

Enumeration strategies

Historically, people were counted by visiting them and assuming all information was correct. Today, we know this isn't always true, so modern methods try to fix mistakes.

During the 2020 U.S. Census, people shared their details by mail, online, phone, or through others. This covered most homes in the United States. Experts check how good this data is.

Some countries use a follow-up survey after the main census to see how many people were missed. This helps make the numbers more accurate for different groups.

A method called short form/long form sends a simple questionnaire to everyone and a detailed one to some people. This gets more information without asking everyone lots of questions.

Some places, like the Netherlands, use records from government offices instead of traditional counting. Others use existing registers, like lists of people and homes, to create census data. France now counts different areas each year, so the whole country is counted every 5 to 10 years. Many European countries mix different data sources for their censuses.

Technology

Censuses have changed with new technology. In 2010, many places used computers to help count people. In Brazil, workers used small handheld devices to find homes. In other countries, people could answer the census online or on paper. In the UK, all census papers are scanned and stored on computers instead of keeping physical copies.

Sometimes new technology can cause problems. The US planned to use handheld computers for its census, but it became too expensive and they changed their plan. Online answers can be helpful, but it’s important to make sure everyone is counted correctly. This means workers still need to visit homes or send forms by mail to confirm addresses. Paper forms are still needed for people who cannot use the internet.

Other ways to estimate populations include using special computer programs called geographic information system (GIS) and pictures taken from far away called remote sensing.

Development

The information from population and housing censuses—like how many people live in an area and their living conditions—is very important for helping countries grow and improve. This data helps leaders decide where to spend money and how to support people better.

Censuses give a complete picture of a whole country, even the smallest parts. This helps leaders plan things like understanding population changes and studying living conditions. They also help find groups that might need extra support, like people with disabilities or those who are poor. Accurate census data helps communities have a say in decisions that affect them.

Uses of data

In the past, people collected information for censuses by hand. This made the data simple. Governments used these results to understand how the population was changing and to make fair decisions. They could also compare their population numbers with other countries.

Later, censuses began to record information about jobs and families. In some places, these old records are now public. They help people learn about their family history. The information can also show how society was structured long ago.

Governments use census data to plan schools and services. With computers, researchers, businesses, and local offices can also use this data to answer new questions and learn more about communities. Today, census data is shared in many formats so that everyone, from students to businesses, can use it.

Census data helps with planning for farming and addressing food security. It also aids research, business decisions, and understanding environmental changes. The data shows important details about farming work and helps track goals related to food security and rural poverty. It provides a foundation for programs and projects at different levels and gives insights into farming and the economy.

Privacy and data stewardship

Census information helps us learn about populations, but it can be misused if personal details are linked to the data. This can happen when individual responses are shared, even if the data is grouped. For example, sharing average income for a small group might let someone guess another person's income.

To protect privacy, agencies use methods like adding small errors to the data or swapping information between similar people. Even with these steps, new technology can sometimes uncover hidden details. One way to prevent this is by using models to show results without sharing the original data. Another option is to share only very general information with the public and keep detailed data safe for the government. Projects like IPUMS work to manage access to detailed data safely, and SDMX helps standardize how data is shared.

Boycotts

Sometimes people have refused to take part in censuses, called boycotts. These can happen for different reasons. Some boycotts happen because groups want special treatment in the census, like in Kenya in 2009 or Myanmar in 2014.

Other times, people use the census to show support for other causes. For example, in the UK in 1911, some groups refused to take part to show they wanted women to have the right to vote. Similar boycotts also happened in West Germany in 1983 and 1987.

World population estimates

Further information: Estimates of historical world population

People have tried to guess how many people live in the world for a long time. One of the first guesses was made in 1661 by Giovanni Battista Riccioli. Later, in 1741, Johann Peter Süssmilch made another guess, and he updated it in 1762. Then, in 1859, Karl Friedrich Wilhelm Dieterici made his own guess about the world’s population.

In 1931, Walter Willcox wrote a book called International Migrations: Volume II Interpretations. In it, he shared a table showing that the world population in 1929 was about 1.8 billion people.

Impact of COVID-19

Impact

In the early 2020s, the COVID-19 pandemic made it hard for many countries to count their people. Out of 155 countries checked by the UNSD, most said their counting work stopped. The sickness made it difficult to meet important dates for counting. Not having enough safety supplies made counting even harder in some places.

The UNFPA asked countries to keep getting ready to count people, even if they had to wait because of the pandemic. Some countries tried new ways to count people, like using the internet, but this took a lot of planning.

Because of this, many countries had to move their counting plans to later dates. The U.S. Census Bureau pushed their important dates four months later. By 2025, only two big countries, India and Nigeria, hadn’t finished counting their people yet. The pandemic also made it hard to count farming-related things in many countries. Over 80 countries said their farming counts were delayed or stopped.

Not having recent counting data makes it hard to make good plans for the country. Scientists can guess what’s happening in the whole country without new data, but it’s harder to know what’s happening in smaller areas. Countries with less money had a bigger problem because they needed to spend more on fighting the pandemic instead of counting people.

Adaptation

Countries tried to use existing records instead of doing surveys to count people. In the US, the counting team used records like income tax filings to count households. Indonesia and Turkey also used their own records to make counting easier.

Because of COVID-19, the United Nations said it was important to use technology to collect data regularly. They believe this helps make better plans for health and housing. More countries are now using new technology to help with counting. The UN plans to support using lists for counting people more in the future, especially for the counting round in 2030.

Modern implementation

Main article: Population and housing censuses by country

In the 2020 round of censuses, from 2015 to 2024, 204 countries or areas did at least one census. Many big places had delays or did not finish because of the COVID-19 pandemic, less money, and people not trusting data collection as much.