An Old Administrative Habit
A census is the systematic counting of a population, usually a country's people and housing. The United Nations defines the essentials as individual enumeration, universality within a defined territory, simultaneity, and a fixed interval, and recommends that countries count at least every ten years.
The word is Latin, from censere, meaning to assess. In Rome the census decided which class a citizen belonged to for both military and tax purposes, and from the middle republic it was usually carried out every five years. It produced a register of citizens and their property, from which duties and privileges followed.
Egypt started earlier and with animals. The first Egyptian count was a cattle count for taxation, taken every two years in the early Old Kingdom, and human censuses only appeared later.
One of the oldest surviving counts comes from Han China in AD 2, which recorded 57,671,400 people in 12,366,470 households. Scholars still consider it quite accurate, though it counted only taxable families, which tells you what it was for.
Knots, Conquest and Cash
The purpose is usually money or manpower, and the methods have varied enormously.
The Inca Empire had no writing system, so census results were recorded on quipu, cords of llama or alpaca hair and cotton with values encoded in knots using a base-ten positional system. Numeric and non-numeric information both went into the knots.
In medieval Europe the best known example is the Domesday Book, undertaken by William I of England in 1086 so he could properly tax land he had recently conquered. In 1183 a census was taken of the crusader Kingdom of Jerusalem, to work out how many men and how much money could be raised against an expected invasion by Saladin.
The first national census of France came in 1328, again mainly fiscal, and estimated the population at 16 to 17 million. Regular censuses were also conducted early in the Caliphate, beginning with one ordered by the second Rashidun caliph, Umar.
Who Counts as Living Where
The hard part of a modern census is not arithmetic. It is deciding who belongs where.
Every person has to be counted once, in exactly one place, and there are three ways to define that place. There is de facto residence, where someone happens to be on census day, and de jure residence, where they legally belong. Permanent residence is a fixed address, such as a family home for a student or a long-term migrant.
These pull in different directions. Where a person actually uses services may matter more than where they slept on one particular night. Students living away in term time, children moving between separated parents, people with holiday homes, and anyone without a fixed address all complicate the count, and second homes regularly cause either double counting or a house wrongly recorded as empty.
Censuses have always worked by finding people where they live, because there is no systematic alternative. Any list you might use to find people is itself likely to have come from a census in the first place.
Forms, Handhelds and Archives
Technology has changed the count without solving its central problem. By 2010 censuses were using handheld devices to locate homes, internet returns alongside paper forms, and scanning that let paper be destroyed instead of archived.
It does not always work. The United States intended to use handheld computers, then costs escalated and the contract was sold to Brazil. Online responses help, but a system that let anyone enter an address without verification would be open to abuse, so households still have to be checked on the ground, and paper forms are still needed for people without internet access.
The data outlives its original purpose. Results are used to measure population change and to apportion representation between areas. In some countries the archives are opened for public examination after several decades, which lets people trace ancestry and gives historians a detailed record that can challenge accepted views of how a society was structured.
It is also political. Canada abolished its mandatory long-form census in 2010 after complaints about intrusive questions, then reinstated it in 2016.
