Rebasing the annual mid-year population estimates for England and Wales

The ONS produces mid-year population estimates annually, which are based on updating from the most recent census. Therefore, whenever results become available from a census, a new base is created for the population estimates. This has implications for historic series, which need to be revised to be consistent with both the past and the most recent census. This article describes the methodology that will be used for this rebasing of the mid-year population estimates following the availability of results from the 2001 Census. Census results also provide a unique opportunity to assess the accuracy of the population estimates that are based on the previous census and this article also describes the approach that will be taken to the assessment of accuracy.     

2011 census coverage assessment and adjustment strategy

Every effort is made to ensure everyone is counted in a census. However, no census is perfect and some people are missed. This underenumeration does not usually occur uniformly across all geographical areas or across other sub-groups of the population such as age and sex groups. Coverage levels in censuses across the world are declining leading to an increasing need to firstly slow the decline through improved enumeration, and secondly in improved methods and data for measuring coverage. In order to achieve the mission critical aims of the 2011 Census, a coverage assessment and adjustment strategy is required. This article outlines the proposed strategy for the 2011 UK Census. The strategy is to significantly improve upon the 2001 One Number Census, and use it as a platform to develop an improved coverage assessment and adjustment methodology. The article also outlines the key areas of innovation for the 2011 strategy and the options that will be considered in order to develop the final strategy. Stakeholder management is also an important part of the strategy to ensure that key users both buy into and understand the methodology. ONS will look to build on the consultation carried out prior to the 2001 Census and widen the user base with which it engages on this important topic.     

Fifty years of United Kingdom national population projections: how accurate have they been?

This article considers the accuracy of the official national population projections made for the UK over the last fifty years The findings take account of the revision to population estimates following the 2001 Census and are largely similar to the findings of a previous review carried out after the 1991 Census. The total population has been projected reasonably accurately but this is largely a chance result of compensating errors in the assumptions of fertility, mortality and net migration. The largest differences between projected and actual populations are for the very young and the very old, while projections of the working age population have been comparatively accurate. Fertility and mortality errors have reduced in more recent projections, while migration errors have grown. However, this may simply reflect the volatility or stability of the respective time-series at the time the projections are made. Changes in estimates of the past and current size of the population (highlighted by the revisions made to population estimates following the 2001 Census) are also shown to play a part in explaining projection error.     

Interim 2001-based national population projections for the United Kingdom and constituent countries

This article describes new 2001-based national population projections which were carried out following the publication in September 2002 of the first results of the 2001 Census. These "interim" projections, carried out by the Government Actuary in consultation with the Registrars General, take preliminary account of the results of the Census which showed that the base population used in previous projections was overestimated. The interim projections also incorporate a reduced assumption of net international migration to the United Kingdom, informed by the first results of the 2001 Census and taking account of more recent migration information. The population of the United Kingdom is now projected to increase from an estimated 58.8 million in 2001 to reach 63.2 million by 2026. The projected population at 2026 is about 1.8 million (2.8 per cent) lower than in the previous (2000-based) projections.     

Estimates of the population by ethnic group for areas within England

This article describes, and provides some initial analysis of, the experimental population estimates by ethnic group for areas within England published by the Office for National Statistics (ONS) in January 2006. The article considers growth and the population structure of each of the ethnic groups identified in the 2001 Census; subnational patterns of change; population turnover; and measures of diversity and segregation, and also provides a comparison of the estimates and corresponding sample-based estimates from the Labour Force Survey.     

Population bases and statistical provision: towards a more flexible future?

In an increasingly complex and mobile society, there is a need for population estimates to be produced on a more flexible basis. Different uses of population data may require information to be output on different population bases, such as where people usually live ('usual residence') or where they are on a particular day ('population present'). This article explores many of the issues associated with defining 'the population'. Following consultation with data users, it outlines recommended population definitions that could facilitate a more flexible approach. Comparisons are made between the output bases produced from the 2001 Census and the more adaptable outputs potentially available in future if the recommended definitions were used.     

Key issues in the quality assurance of the One Number Census

As part of the 2001 Census, the One Number Census project estimated and adjusted the Census database for underenumeration. As a result of the highly innovative One Number Census and the Quality Assurance process it encompassed, it was also ensured that robust results could be obtained for each local authority area. This article examines some of the issues and analyses that were undertaken as part of that assessment of the 2001 Census population counts for England and Wales. The article firstly highlights the key issues surrounding the implementation of the 2001 Census fieldwork. The article then explores the 2001 Census results through a series of demographic analyses to illustrate the sorts of issues investigated during the One Number Census Quality Assurance process itself. These analyses look at the patterns contained within the results, and comparisons with key alternative sources of population counts. Overall, these in-depth analyses and investigations provide further credence to the plausibility of the One Number Census results.     

The 2011 Census: a proposed design for England and Wales

This article sets out the proposed design for a 2011 Census in England and Wales, as part of the ONS's proposed future population statistics system. The design draws on experiences from the 2001 Census and changes in available technology, and takes account of the many comments received from respondents to a Discussion Paper published in October 2003 and of views expressed at a joint ONS/RSS Conference on 11-12 November 2003. The article highlights the statistical and operational strategic aims for the 2011 Census, the major changes proposed from the 2001 approach, and gives an indication of the benefits and risks. It is important to note that this design remains a proposal at present, the elements of which will be subject to further research building on comments received during consultation and a detailed testing programme over the coming years.     

A new area classification for understanding internal migration in Britain

This article details the development of a new area classification for Britain based on internal migration variables taken from the 2001 Census. An explanation of why general-purpose area classifications already in existence are not ideal for internal migration analysis is provided, before an account of the construction of the new classification is given. The latter involves justification of the choice of variables, explanation of the methodology adopted and presentation of the final typology.     

One-parent families--and the dependent children living in them--in Great Britain

This article provides updated final estimates of the number of one-parent families, and of the number of dependent children living in them, from 1995 to 1997, inclusive, together with provisional estimates for 1998 to 2000. The existing methodology has been extended so that additional alternative estimates have been generated from which to assess the "best estimates". The number of one-parent families in Great Britain is provisionally estimated at 1.75 million in 2000, and the number of deppendent children living in those families as 2.9 million.     

The Census Coverage Survey--the key element of a one number census

The key measure of census quality is the level of response achieved. In recent censuses worldwide this level has been in the high nineties per cent. This was also true of censuses in Britain in 1991 (98 per cent). However, what was particularly noticeable about the census in Britain in 1991 was the differential response rate and the difficulty in effectively measuring this. The One Number Census programme was set up in the UK to research and develop a more effective methodology to measure and account for underenumeration in the 2001 Census. The key element in this process is the Census Coverage Survey--a significantly larger and redesigned post-enumeration survey. This article describes the planning and design of the survey with particular emphasis on the implementation of the proposed field methodology in practice. It also provides a high-level overview of the success of the survey.     

Methodology used for producing ONS's small area population estimates

The Office for National Statistics (ONS) has set up a project to investigate the feasibility of producing postcensal small area population estimates on a nationally consistent basis for England and Wales. Following on from earlier investigations to identify potential data sources and methods, ward and Super Output Area population estimates have been released as 'experimental statistics'. This article covers the methodology used to produce these estimates, feedback from user consultation with the initial estimates released at ward level, summary statistics on the population estimates for mid-2001 to mid-2003, and future developments for small area population estimates.     

Developments in census taking in the last 25 years

This article looks back at censuses of population conducted since 1971. It identifies some of the most important changes with respect to questions asked, methods of collecting the census and the production of outputs. In particular it discusses the introduction of the ONS Longitudinal Study following the 1971 Census and the Samples of Anonymised Records following the 1991 Census. This article outlines some of the changes being introduced for the 2001 Census, including methods of dealing with under-enumeration and improved access to outputs.     

Quality assuring the 2011 Census population estimates

The 2011 Census is a once in a decade opportunity to gain an accurate picture of the population. Quality assurance of the census population estimates is vital to ensure data robustness and that users have confidence in the results. This article provides a detailed summary of the methods being used to achieve this quality assurance, including the processes and adjustments.     

The development of a 'Postcode Best Fit' methodology for producing population estimates for different geographies

A 'Postcode Best Fit' methodology has been developed by the Office for National Statistics to produce population estimates for a range of different geographies which are entirely consistent with each other, regardless of whether or not the estimates for one geography can be aggregated to produce estimates for another geography. This article describes the Postcode Best Fit methodology, its evaluation, limitations with some of the data sources used with the method, the application of the method for producing population estimates and case studies describing how the method has been used to produce bespoke population estimates to meet specific user requirements.     

Reviewing the mid-year population estimates in Northern Ireland

This article describes the thorough review of the mid-year population estimates for Northern Ireland. No major problems have been identified with the current estimates but a number of enhancements have been put in place. The new figures are consistent with a number of administrative data sources. The overall effect has been to revise the estimated Northern Ireland population by about 6,000 people, or between 0.3 and 0.4 per cent, for each year between 1991 and 1997. The revised estimates for Northern Ireland have been incorporated into the population figures for the United Kingdom included in the Table section of this issue of Population Trends.     

Opportunities for new research using the post-2001 ONS Longitudinal Study

The addition of 2001 Census data to the ONS Longitudinal Study extends the range of research topics that this unique data resource can support. Census questions on religion, care-giving and self-rated health that were asked for the first time in 2001 and the repetition of 1991 questions on limiting long-standing illness and ethnicity raise opportunities for new longitudinal investigation in these areas. This article describes how new 2001 methodologies including data imputation, the One Number Census and de jure enumeration affect the LS database. The support service for existing and prospective LS users is described.     

A cross country review of the validation and/or adjustment of census data

This article reviews existing procedures employed by various countries in the evaluation of, and/or adjustment, either of census data, or of population estimates based upon census data. The work was carried out to ensure all potential demographic techniques are considered by the ONS for the post census evaluation process of the 2011 Census.