What is a population census?

A population census periodically counts each person in a country – or any defined geographic area – on a specific day and records where they live, what they do and other relevant personal characteristics. Some advantages of a census are:

• A census generates essential population data to calculate multiple health and demographic indicators
• Health sector specialists can use census results to plan and monitor health interventions
• Combining census results with health system information provides insights into local health conditions
• The census provides the sampling frame for health-related household surveys

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How have censuses evolved?

The earliest examples of true censuses are the 1666 census of Quebec (New France) and the 1703 census of Iceland then a dependency of Denmark. These met the key requirements of a census: individual enumeration, complete coverage throughout a defined territory, and undertaken at the same time.

From the early nineteenth century statisticians began to standardize census methods. Under the leadership of the Belgian scholar Adolphe Quételet they established: the  type of population to enumerate; frequency of census taking once in ten years preferably in years ending in zero; reference to the census date; use of the family or household to identify individuals; and use of specially trained enumerators and a questionnaire.

In 1980, the United Nations published the Principles and Recommendations for Population and Housing Censuses which established the link between housing and population censuses, their linkage with other types of censuses, and promoted a regional approach.

During the 2010 World Population and Housing Census programme, 214 countries or territories conducted a census covering 97 per cent of the estimated world population; twenty-one countries or areas did not participate.

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Why undertake censuses?

Because censuses:

• Support the democratic nature of society and promote equality.
• Provide rich cross-sectional material about changing demographics- demographers use the data to create life tables from which they estimate life expectancies, fertility and adult mortality rates.
• Inform citizens, scientists, non-governmental organizations and private sectors with basic information about population structure.
• Assist governments determine electoral boundaries and representations and to allocate resources.
• Measure the flow of international migration (if recorded).
• Provide sectors with data to plan and deliver services, for example to allocate health and social services.
• Provide comparable global   geographic distributions of human resources if the International Standard Classification of Occupations is used.

Population data from censuses undertaken for 2000-2010 provided the denominators for most indicators for the Millennium Development Goals

Population data from censuses undertaken for 2020-2030 will be essential to measure indicators for the Sustainable Development Goals

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Four conditions for success

• The country is at peace with political, social and environmental stability.
• The government has enacted required legislative and administrative arrangements and identified sufficient and timely funding.
• A critical mass of qualified professionals is available and able to plan and execute the census.
• The population is willing to provide the required information.

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Types of census data

The census office will consult government agencies, the private sector, non-governmental organisations, professional,trade and religious organisations and the public on the potential content of the census questionnaire. But its goal is to keep the questionnaire short.

Core topics

Core topics are obligatory questions that should be included in the census. For the 2010 World Census Programme, core topics were: age; sex/gender; marital status; relationship to the household head; place of usual residence; educational status; occupation; number of live births; date of the last live birth; children still alive; deaths in the household in the last 12 months; and disability status.

Non-core topics

Non-core topics can be included at the discretion of the census office and the government. Typical non-core topics for the 2010 World Census Programme were: age at first marriage; date of and duration of the first marriage; country of birth of father/mother; the age of mother at first birth; maternal or paternal orphanhood; time in current employment; and distance to place of work

The United Nations Statistical Division recommends these topics to include in a census.

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Undertaking a census

Preparing for a census

During the preparatory phase, the census office prepares all administrative, technical and operational activities including, training of field staff, setting-up data management and comprehensive quality control systems. The office must develop a communication and publicity programme that reflects diversities of culture and language of the population.  Two essential tools are a multi-lingual website and help-line.

Field operations

The field operations phase includes preparatory activities for enumeration, the enumeration proper, and field verification processes. These activities include updating the census cartography, identifying dwellings, preparing dwelling and household lists, enumerating all persons in all households, verifying completeness and correctness of the enumeration, and carrying out a post-enumeration survey to establish the level of census coverage.

Options to enumerate the population include:

• Face-to-face personal interviews, in which an interviewer visits each household and completes the questionnaire;
• Self-completion of the questionnaire, using the mail-out-mail-in procedure, in which the questionnaire is delivered to each household (address) and the occupants complete the questionnaire themselves;
• Telephone interviews in which the household responds to the census questionnaire by telephone, either through its own initiative or after being contacted by the census office; or internet interviewin which the occupants of the household complete an electronic version of the questionnaire.
• Computer-assisted personal interviewing (CAPI) packages can verify the correctness of answers during interview instantly producing high-quality data, reducing or eliminating need for lengthy data management after field operations. These data are immediately available for analysis, reducing processing time and possibly also the total census cost. However, unless special provisions are made in the software  it is not possible to  assess the quality of the original responses.

Data preparation, evaluation and tabulation

During the data preparation, evaluation and tabulation phase, specially trained staff manually process collected data and self-completed questionnaires at data centres; they transform the data into electronic formats, mostly using automated electronic scanning. They verify all data, whatever the collection format, for completeness, correctness and consistency, using data control techniques focussing particularly on age data. the following software packages are freely available to handle the data:

• CSPro (Census and Survey Processing System) is a public domain software package used by hundreds of organizations and tens of thousands of individuals for entering, editing, tabulating, and disseminating census and survey data.
• MORTPAK is a software package for demographic measurement in developing countries, with special emphasis on mortality measurement.
• PAS (Census Population Analysis System) is a set of Microsoft Excel workbooks developed by the U.S. Census Bureau containing frequently used procedures and methods in basic demographic analysis.
• REDATAM (REtrieval of DATa for small Areas by Microcomputer) is a friendly and interactive computer system that facilitates the processing, analysis and web dissemination of information from population censuses, surveys, administrative records, national/regional indicators and any other sources.

The results are presented in tabulations for the different levels of administrative units by census theme and age and sex. Sometimes separate reports are issued bor specific census themes.

Population pyramids show composition by sex and age and illustrate the effects of changes in fertility, mortality and migration over time when several censuses are compared.

The WorldLifeExpectancy website illustrates dynamically changing population pyramids over time for countries of the world

Maps vividly display any census findings by location. The US Census Bureau, for example, provides interactive maps for its census results

Dissemination

During the dissemination phase, the census office disseminates the results and output of the census to the government and society in a number of forms:

• The preliminary report, issued soon after completion of field work, tabulates population size by sex and age for the country and its main administrative subdivisions.
• The final report contains detailed tables on the population and its characteristics for administrative subdivisions.
• The administrative report provides a full account of the census process, its challenges and solutions and includes copies of all relevant documents.
• The census atlas presents results graphically especially as maps. With information from more than one census, atlases can show changes in population characteristics over time.
• The updated census cartography serves as the national master sampling frame for surveys and for other censuses such as agricultural or industrial censuses.
• Updated inventories are lists of localities, inhabited or not, building types, dwellings and households with relevant geographical identification information.
• Metadata provide detailed and complete documentation of methods, techniques, procedures, variables, questions, response categories and coding instructions used in the census
• Depersonalised anonymised data files of the population, or a sample, can be made available for third-party research analyses.
• Historical completed questionnaires can become publicly available for genealogical and family research after a suitable period of time.

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Challenges

• The census can be a burden on low- and middle-income countries which often lack financial, infra-structural and human resources to undertake it, especially as donors have cut their funding to support census taking.
• There can be international and national pressure to modify census procedures, either by changing the type of population to be enumerated, or to use sophisticated equipment without consideration of the necessary trained human resources.
• The census operations and quality maybe compromised by having excessively lengthy questionnaires which can strain limited resources and extend the training of census workers
• Unanticipated security issues and environmental developments may affect the proper execution of the census activities.

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Future developments

Two alternatives to the census are in use:

• A rolling census, only used by France, is a system of continuous sub-national rotating sample surveys. The rolling census cannot produce estimates for the whole country at a specific moment or a census reference date.
• A register-based census is only possible when a country efficiently maintains and coordinates a system of registers linked to a national population register. During the 2010 World Census Programme, 15 countries used a register-based approach as their sole or main source of census data. Different countries follow different procedures, and there is no standard methodology for a register-based census.

The census provides detailed, complete and verifiable information of a large numbers of variables for different levels of the national territory. The so called-register-based censuses do not have procedures to verify or establish the completeness of the coverage of the total population of a country or territory. The scientific principles that the census adheres to are well-established; it is flexible in incorporating new technologies and it is capable of enumerating rare populations or personal characteristics and measuring emerging phenomena.

Until there are alternative procedures to produce the same population information with identical quality and completeness, the census will remain the preferred way of obtaining periodic information on national and sub-national populations.

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