How information is collected and processed

The last census was carried out in 2001. A census form was delivered to every household, establishment, or to people living anywhere else, by a field force set up throughout the country.

The forms were designed for self-completion by form-fillers and to provide information relating to census day - 29 April 2001. Most forms were then posted back to temporary local offices and the remainder collected by the field force. Census forms are available to view from the Related links section.

The form for a household in England asked questions which collected information on household accommodation, relationship, demographic characteristics (for example, sex, age, marital status), migration, cultural characteristics, health and provision of care, qualifications, employment, workplace and journey to work. In Wales there was an additional question on the Welsh language. The forms for people who lived communally, such as homes for the elderly, collected information on each person. To view a list of 2001 Census topics, see related links.

Everyone in the country was under an obligation to complete a census form, and a limited number of people were prosecuted for failing to comply with this obligation. A major publicity campaign helped raise awareness of the census and its importance. There was also a special programme to help various groups who might have difficulty with the census.

The 2001 Census forms were designed to be fed at high speed through scanning machinery which captured all the ticked responses and held written answers in digital forms. Written answers were coded into categories either by automatic systems, which recognise terms given in response to questions, or by manual coding. The capture of data for around 60 million people and 24 million households in the UK was scheduled to take around ten months from a start in June 2001.

Once information from the forms had been electronically captured, the paper forms were pulped and recycled. The digital images of the forms were transferred to microfilm to be made available as public records after 100 years.

Data capture was contracted out to private suppliers. The census organisations then processed the data before loading them into a database from where the results were drawn. The main processing phases were:

'edits' to ensure that data are consistent
'imputation' to insert the best estimate of a missing response
statistical adjustment to allow for any shortfall detected in a follow up survey of census coverage
management of data quality throughout
high speed tabulation software producing aggregated statistics from the output database of individual records for delivery to users in print or electronic form

When the database was complete simple figures, such as the total number of people living in a place, were prepared on every topic. The great value of this 'snapshot' of the population is that combinations of circumstances, such as the number of single parents with young children living in the upper stories of high rise housing, are collated.

Each stage of the census planning, field and processing operations was evaluated and reported in a series of online notes. For more information, see 2001 Census review and evaluation section.

The enumeration and processing methods for the 2011 Census are likely to be a mix of both innovative and traditional methods. For more information, see Processing the information for the 2011 Census in related links.