A census of the world population is taken every ten years

A census is taken in the various countries of the world roughly every ten years. In China, six million enumerators were needed in 2010 to go around every household to collect information on forms. In Finland, on the other hand, the population census at the turn of last year was hardly noticeable. Since 1990, Statistics Finland has been carrying out censuses directly based on administrative registers without troubling the citizens, but in most parts of the world, censuses still rely on the conventional method of interviews.

Following a UN recommendation, an effort is made to conduct population censuses always in years ending with a ten, while in practice the time of the census may vary in different countries. The first data for the census of 2010, for example, had already been collected in 2005 in some countries, whereas in others they will not be collected until 2014.

In April 2011, an estimated 79 per cent of the world population had been enumerated, and a census had been carried out in 134 countries. Only six states in the entire world have no plans to carry out a census in the near future.

Regular censuses can only be taken in wealthy industrialised countries. A lack of resources alone prevents developing countries from carrying out frequent censuses. However, a census has been taken in all countries of the world at least once since the 1950s.

A census describes the entire population

The primary purpose of a census is to describe the population living in the country at a certain moment in time: the number of population, population structure and its geographical distribution. In addition, censuses collect information on the living conditions of the population, educational structure, families and households, number of children, migration, economic activity, occupations and branches of industry. Information about buildings and dwellings is often also collected in connection with censuses.

Many countries also produce this type of information not only in connection with the actual censuses but also at other times. In Finland, population statistics are compiled annually based on the Population Register, and taking an actual census is thus unnecessary purely to establish the number of population. Various types of sample-based statistics can also be used to produce information about the economic activity of the population at national level. The Labour Force Survey, for example, is a sample survey repeated on a monthly basis, which produces readily available information about employment and unemployment.

Sample surveys do not produce accurate information on various geographical areas, however. In many countries, a census is the only statistics producing accurate information by geographical area. The key advantage of a census is that it covers the population living in the entire country, and information can also be obtained from very small geographical areas.

Majority of censuses conducted by interviews

In most countries of the world, censuses are still carried out following the customary method of collecting data directly from the population, either by interviews or by a form that is mailed to households. For example in China, India, Russia and many other countries with a large population, a conventional census is conducted, where enumerators go round from door to door, recording data on a form.

Conventional interviews are used especially in developing countries where the poor literacy skills of the population make it impossible to collect information by mail. In addition, an incomplete address register and poor infrastructure may prevent countries from using a mail survey.

Taking a census in Kosovo, spring 2011

In a mail survey, a census form is sent to each household. An individual form is filled in for each member of the household, and the forms are mailed back to the statistics authorities. In addition to literacy, the population must have an educational standard ensuring that the questions and concepts can be understood. Until the census of 1960, interviewers only were used in the United States. The reasons for this included the fact that the English proficiency of immigrants living in the country was not expected to be good enough to fill in the census form without assistance.

Interviewers are often used in addition to a mail survey. In the 2010 census in the United States, enumerators collected the information from those who failed to return the form by mail. Some countries have also introduced an online form in addition to the customary printed one. Using the online form cuts down on the costs of data collection and improves data quality.

The first direct census in Finland in 1950 was also carried out by interviews. In the censuses of 1960 and 1970, municipal census committees were used. After this, censuses by mailed questionnaires were introduced. From 1970, register data were used in addition to information collected directly until the year 1990, when the census was finally carried out exclusively based on registers.

Register-based censuses becoming more widespread

Denmark was the first country in the world to conduct a completely register-based census as early as 1980. Finland was the next one to follow suit in 1990. Using existing administrative material and registers in censuses and statistics has become more widespread in the last few decades. The 2010 census is exclusively register-based in the Nordic countries, Austria and Slovenia.