Select Committee on Treasury Eleventh Report

4  Difficulties in counting the population

Problems with the 2001 Census

54. David Coleman, Professor of Demography at Oxford University, argued that "the UK has not had a satisfactory census since 1981". He noted that

Census totals are compared with the annual population estimates for the same year derived from the updating of the previous census by the intervening total of birth and deaths and net migration. In theory the two totals should agree when the census total (usually April) is adjusted to fit the population estimate (usually mid-year). It is, however, a moot point to decide which of the two should be regarded as the benchmark, and neither will be exactly accurate. Both the censuses of 1991 and 2001 deviated considerably from the corresponding population estimate and, despite every effort, each has turned out to be in error, or at least remain controversial. In 1991 the estimates rolled forward from 1981 were preferred (erroneously) over the census. In 2001, the census was preferred (only partly erroneously) over the estimates rolled forwards from 1991.[67]

55. He also pointed out that "the census of 1991 generated a population (49,890,000 in England and Wales) over one million fewer than that expected from the 1991 population estimates updated from the 1981 census (51,105,000); the so-called 'missing million'. For various reasons, particularly its finding of an implausible low sex ratio of 94.1 males per 100 females, the census, not the estimates, was deemed to be in error, and various upward adjustments were made".[68]

56. In order to avoid the repetition of these problems and the difficulties caused, notably to Local Authorities, of a number of differing estimates of population for the same year, efforts were made to ensure the 2001 Census was accurate and in-line with the expected population estimates for 2001. Census methodology included an exceptionally large post-census enumeration survey, the Census Coverage Survey, of 320,000 households, using a sampling methodology separate from that of the census. The 2001 Census methodology was intended to avoid the defects of its predecessor, the smaller Census Validation Survey from 1991 (6,000 households), whose methodology followed that of the census and therefore tended to duplicate its errors, those missing in the census also being missed by the survey.[69] Professor Coleman pointed out that "the kinds of households that are likely to escape or evade the census are also likely to escape or evade any surveys for the same reasons".[70]

Accuracy of the mid-year population estimates

57. The accuracy of the mid-year population estimates is dependent on the quality of data available to measure components of population change (births, deaths and migration). Migration, both internal and external, is the hardest component to measure. [71] Of the data sources currently used to calculate population estimates:

  • The Census arguably provides the most reliable base and set of distributions;
  • Birth and Death registrations are considered to reflect accurately numbers of events occurring in the United Kingdom;
  • Internal migration data are reliant on people registering change of address with their doctors promptly after a move; and
  • International migration is difficult to estimate, but use is made of available sources.

There are some additional data sources which are used to estimate the population in some local areas:

  • Counts of 'long term' prisoners;
  • Counts of boarding school pupils;
  • Estimates of the number of UK armed forces; and
  • Estimates of foreign (American) armed forces. [72]

58. Mr Blake-Herbert, Director of Finance, Slough Borough Council told the Sub-Committee that "at the last census Slough had the ninth fastest growing population in the country, despite having the fourteenth worst returning rate for census forms. Since then the mid-year estimates initially showed us having the second fastest declining population in the country." He was concerned that the mid-year estimates had inaccurately estimated the population in Slough. He explained that "the ONS will not use the child benefit data for the number of children in an area because they acknowledge [child benefit data] is a huge under count. Actually, there are more children receiving child benefits in Slough than the Office for National Statistics currently say live in Slough". He stressed that "this is not just about international migration; it is about migration within the country. ...the statistics are not keeping pace, and because of that we are not able to provide the right services".[73]

59. The Bank of England noted that there was a great deal of uncertainty surrounding the current mid-year population estimates which related to the measurement of net migration. Official estimates of international net migration are primarily based on the International Passenger Survey (IPS), a survey of individuals passing through the main UK air and sea ports and the Channel Tunnel. The ONS supplement the IPS with administrative data on asylum seekers and their dependents, and estimates of the migrant flow between the United Kingdom and the Irish Republic; and the ONS make adjustments to account for those whose intended length of stay changes.[74]

Counting a migrating population

60. The National Statistician told the Sub-Committee that within the UK migration was estimated based on samples of the population and therefore she was "less confident in statistical terms about that".[75] Several of those who submitted evidence to the Sub-Committee argued that during the last ten years, migration (both international and internal) had been the dominant influence on population change. [76] The Statistics Commission noted that "for those areas which present the greatest challenge statistically, much of the challenge relates to migration (international and internal)".[77]

61. The Statistics Commission told the Sub-Committee that it had pressed for improvements to migration estimates since 2003, arguing that there were potentially large economic costs from not knowing with sufficient precision the size and geographic distribution of the population.[78]

The Office for National Statistics (ONS) has given priority to improving migration and population statistics. It is making improvements to the methods and data sources used to estimate the population at national and local levels during the inter-censal period. The ONS argued that these improvements were needed to minimise the risk of divergence between the rolled forward mid-year population estimates and the 2011 Census-based population estimates, and to better understand the differences that remain. [79]

62. The provision of accurate information about how many people are present within the country and where they are located is essential to effective policy-making and the effective delivery of services. Society is becoming more mobile and the information held electronically about events, persons and services by government agencies and other bodies has substantially increased. We require the Statistics Authority in response to this Report to set out the steps it will take to utilise and better link data held by the Government and by local government in order to provide a more accurate picture of the population within this country.


63. In 2005 (the last year for which detailed data is available), the IPS statistics on migration were based on interviews with 2,965 people who entered the United Kingdom and 781 people who left.[80] This was a very small sample and suggests why there were large uncertainties surrounding the official migration numbers. In addition, it is difficult for the survey to keep pace with the dramatic change in the pattern of arrivals seen in recent years, for example in the change in movements between the UK and A8 countries. In 2005, only 94 citizens of the A8 countries were interviewed.[81] Approximately 90% of all migrant interviews took place at Heathrow, with very few interviews taking place at other airports such as Stansted and Luton. The IPS may also fail to accurately measure international migration as the survey was not designed to measure net migration, but was designed to capture tourism and business travel; participation in the survey is voluntary and immigrants may be less likely to respond (perhaps because of language barriers); and the survey asks about intentions on arrival, not what people actually do. [82]

64. Professor David Coleman pointed out that

As a survey, the IPS is subject also to non-sampling errors including non-response. The questions also focus on intentions (about length of stay and where migrants will live). These are often not realised, or may subsequently change. To estimate the amount by which actual and intended length of stay differ, new IPS question were introduced in 2004, to be asked of those interviewed at the end of a stay. Based on the answers to these questions, adjustments have now been made to national estimates of long term migrants. For 2006, this added 28,000 to the estimate of net migration. [83]

65. The Bank of England pointed out that there was a risk that current population estimates could be under-recording the true population. The Bank noted that "other sources of administrative data suggest that net migration from the A8 countries may be higher than is recorded in the latest population estimates". The most recent official population data suggested that between mid-2004 and mid-2006 there was a gross inflow of 151,000 A8 citizens into the UK whose intended length of stay was at least one year, and a net inflow of 131,000. However, between May 2004 and June 2006, 433,000 A8 nationals registered for work under the Worker Registration Scheme (WRS) and had their application approved. Over the same period, just over 400,000 National Insurance Numbers were issued to people from A8 countries. The Bank explained that the reconciliation between the population data and the other sources required that either a very high proportion of those registering for the WRS or applying for National Insurance Numbers were temporary migrants, or that the population data was under-recording the true level of net migration from the A8 countries. The Bank of England noted that weight should be attached to both hypotheses. [84]

66. The Inter-departmental Task Force on Migration Statistics recommended that to improve information collected on migrants at ports, the ONS initiate a review of port surveys. An interim report of the review was published on the National Statistics website in October 2007.[85] The report recommended that survey takers should undertake additional shifts at Manchester, Stansted and Luton from 2008 to improve the sample for migration purposes. The ONS told the Sub-Committee that this recommendation would be implemented. Further changes will be recommended for 2009 in the final report. Improved information from a port survey will be used in combination with other sources, such as the new enlarged household survey starting in 2008 and the 2011 Census. [86]

67. The International Passenger Survey was designed to provide data primarily for tourism and business travel purposes. It is now called upon to play a central role in estimating international migration. It is clear from the evidence we have received that the Survey is not fit for this new purpose. We recommend that the Statistics Authority replace the International Passenger Survey with a new Survey that is more comprehensive and more suited to the accurate measurement of international movements affecting the size of the resident population of the United Kingdom.


68. Inaccurate or misunderstood population estimates can potentially invalidate policy decisions, and reduce the value for money of public services. [87] The Statistics Commission cited examples of cases where population figures were not "fully fit for purpose", such as statistics estimated in relation to inner city areas and statistics used as specific indicators of need to spend on public services that are not closely related to resident population numbers. [88]

69. The Sub-Committee received 24 submissions from Local Authorities and Council which argued that the UK was not measuring population changes well at a local level and existing government data that could help local government researchers and policy staff was difficult to access.[89] Sir Simon Milton, Chair of the Local Government Association and Leader of Westminster City Council explained that Local Councils found the current methods of estimating internal migration within UK unsatisfactory, "because, fundamentally, councils are finding that they are having to service populations which their funding regimes do not recognise".[90]

70. Some Local Authorities, such as Manchester and Westminster were directly and adversely affected by enumeration problems in the 2001 Census. Although total population figures were revised, the published census counts remained unchanged with the result that the figures were known to be unreliable. Professor Martin pointed out that such demonstrably inaccurate basic population counts could call into question the entire edifice of resource allocation decisions, target-setting, prevalence rates and area profiles which were essentially reliant on such key population data.[91]

71. Based on the evidence we have received, it is evident that there are substantial problems in generating accurate population estimates in some Local Authority areas. The current methods of estimating internal migration are unsatisfactory and lead to decisions on the allocation of funding to Local Authorities being based on inadequate information. The Statistics Authority should establish as an immediate priority the provision of local population statistics that more accurately reflects the full range of information available about local populations and the effects of internal migration.

Short-term migration

72. Estimates of international migration used in the mid-year population estimates are based on the United Nations definition of a long-term migrant: someone who moves to a country other than that of his or her usual residence for a period of at least a year (12 months), so that the country of destination effectively becomes his or her new country of usual residence. From the perspective of the country of departure the person will be a long-term emigrant and from that of the country of arrival the person will be a long-term immigrant.[92]

73. Short-term international migrants are not included in the usually resident mid-year population estimates. The Sub-Committee received written evidence from a number of statistics users stating that the 'usually resident' definition of population did not fully meet their needs because short-term international migrants were not included.

74. The Statistics Commission stated:

Short-term migration is important as it is not normally included in the official figures for migrants (which, following international practice, only counts as 'migrants' those intending to stay for more than a year). But, perhaps particularly following EU enlargement, short-term migration has the potential—and already the reality in some areas—to have significant implications for local services. [93]

The Local Government Association reported that there was widespread concern among Local Authorities that the official population estimates did not reflect experience at local level. It stated:[94]

A particular difficulty with the usually resident definition concerns the treatment of migrants. The current system only counts those migrants who state the intention to remain in the UK for at least 12 months at the time of entry. There is considerable evidence that many migrants come for a shorter period (but nevertheless make demands on services); and that others stay for longer than they had originally intended.

75. London Councils argued that the exclusion from population estimates of migrants who enter Local Authority areas and use public services, but who did not settle for a year or longer, was clearly penalising areas such as London which had high levels of migration.[95]

The definition of a resident from overseas for population purposes, currently defined as a migrant intending to reside in the UK for at least a year, has become increasingly important over the last few years due to increasing numbers of short-term migrants in the UK. This is a particular issue for London, as demonstrated by recent experimental statistics published by the ONS which suggest that 40% of all short-term migrants live in London. Therefore, the current definition of a resident fails to meet the need for population statistics for funding purposes because they exclude a significant proportion of people who are living in the capital. This means that Local Authorities that experience influxes of short-term migrants do not receive funding for people who are using their services. Thus, there is an urgent need for estimates of short-term migration at the Local Authority level, or for them to be included in existing population statistics.

76. The Sub-Committee received written evidence from many Local Authorities across the country expressing similar concerns about the impact of increased internal migration on resource allocation. They also explained the need for both long-term and short-term population estimates in order to plan service delivery.

77. One of the recommendations put forward by the Inter-departmental Task Force on Migration Statistics was to produce estimates of the number of short-term migrants.[96] In January 2007, the ONS published a report on the feasibility of estimating short-term migration[97], followed by feedback[98] in April and experimental estimates for England and Wales in October.[99]

78. The ONS research report on short-term international migration published in October provides estimates of short-term migration at the national level for England and Wales. A regional split of London/non-London is also provided. Estimates of short-term migration are based on the United Nations definition of a short-term migrant: someone who moves to a country other than that of his or her usual residence for a period of at least 3 months but less than a year (12 months) for study or employment purposes.

79. Taking the ONS's recent work on Short-Term Migrants as an example, there are estimated to have been only 43,000 Short-Term Migrants in England and Wales in the year up to June 2005, with just 16,000 of these in Greater London. These estimates are based on the International Passenger Survey, and appear to many users to be implausibly low when contrasted with administrative data such as National Insurance Numbers, which in 2005-6 recorded 235,640 new National Insurance Number registrations in Greater London alone. Many of the latter no doubt returned to their countries of origin within 12 months, but having the National Insurance Number counts available provides a valuable additional insight.[100]

80. The recently published ONS experimental short-term migration estimates, although a welcome development towards estimating the true population, do not reflect the scale of migration in London that is evident from other administrative sources of data e.g. National Insurance registrations. Further work would be needed to improve these estimates and the sample sizes of international migrants entering the country need to be improved considerably to place greater confidence on the data.[101]

81. ONS recognised the need to produce sub-national short-term migration estimates and was currently investigating the feasibility of producing estimates of the number of short-term migrants at local level. The Sub-Committee received evidence stressing the importance of ONS being allocated sufficient resources to produce robust local estimates of short-term migration.[102]

82. The Statistics Commission stated that the accuracy of population estimates varies geographically across the country. Urban areas for example, characterised by large student and migrant populations, multi-occupancy households and 'part-time' residents, are subject to greater uncertainty in their population estimates than rural areas. The Statistics Commission suggested that ONS carry out a series of Local Authority case studies to investigate and create a good practice guide on different data sources available.[103] Mr Dugmore argued that the accuracy of different administrative registers should be compared with the 2011 Census in view of the possibility of using administrative and population registers in counting the population in the future.[104]

83. Official mid-year population estimates, based on the 'usually resident' definition of population, do not include short-term migrants. Such estimates do not fully meet the needs of Local Authorities and commercial users who are also interested in, for example, short-term migrants as well as day-time and week-day populations. We recommend that the Statistics Authority investigate the feasibility of producing population estimates based on different measures of population, such as estimates which include short-term migrants and estimates which include the day-time population of Local Authorities.

84. We are seriously concerned about the reliability and validity of ONS estimates of short-term international migrants. Evidence from administrative data sources such as the National Insurance Number register suggests the ONS estimates do not reflect the scale of short-term migration in England and Wales. We recommend that the Statistics Authority examine the feasibility of producing estimates of short-term migration at sub-national level, using the successor to the International Passenger Survey that we recommended earlier and a greater range of administrative data.

85. We further recommend that the Statistics Authority continue the ONS's work with Local Authorities and carries out a series of case studies to identify alternative administrative data sources. These include the National Insurance Number register, GP lists, other health service lists, council tax records, and various registers on children and school children. Although we recognise that different areas have different problems associated with counting the population and administrative registers, we recommend that the Statistics Authority produce a best practice guide.

67   Ev 191 Back

68   Ibid. Back

69   Ibid. Back

70   Ibid. Back

71   Ev 209-212 Back

72   Ibid. Back

73   Q 147 Back

74   Ev 291 Back

75   Q 256 Back

76   Ev 33, 44, 53, 59, 64, 69-108, 124-132, 142, 150-157, 166-183, 198 Back

77   Ev 33 Back

78   Ibid. Back

79   Ev 215 Back

80   Ev 106 Back

81   Ev 289-292 Back

82   Ev 291 Back

83   Ev 209-212 Back

84   Ev 291 Back

85   Interim report, Inter-departmental Task Force on Migration Statistics, October 2007, National Statistics, Back

86   Improving Migration and Population Statistics Project (IMPS), National Statistics, Back

87   Ev 33 Back

88   IbidBack

89   Ev 44, 53, 59, 64, 69-108, 124-132, 142, 150-157, 166-183, 198 Back

90   Q 140 Back

91   Ev 24 Back

92   Report of the Inter-departmental Task Force on Migration Statistics, ONS, December 2006 Back

93   Ev 34 Back

94   Ev 110 Back

95   Ev 70 Back

96   Report of the Inter-departmental Task Force on Migration Statistics, ONS, December 2006 Back

97   Short-term migration feasibility report, ONS, January 2007 Back

98   Summary of feedback received to short-term migration feasibility report, ONS, April 2007 Back

99   Research report on short-term migration, ONS, October 2007 Back

100   Ev 22 Back

101   Ev 94 Back

102   Ev 119 Back

103   Q 53 Back

104   Q 56 Back

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