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					2008 Mid-Year Population Estimates
Frequently Asked Questions
August 2009

Q1 What do the mid-2008 population estimates tell us about population

Latest figures show that in the year to mid-2008 the UK population increased by 408,000 to
61,383,000 (an increase of 0.7 per cent), compared to an increase of 388,000 in the
previous year (0.6 per cent). In the seven years since 2001, population has increased by an
average 0.5 per cent per year.

Increases in numbers of births, decreases in numbers of deaths and changes in the pattern
of international migration into and out of the United Kingdom have all contributed to
population change. For the first time in nearly a decade, natural change (the difference
between births and deaths) has overtaken net migration as the main contributor to
population growth over a 12 month period. In the year to mid-2008 natural change
accounted for 54 per cent of all population growth in the UK, compared to 48 per cent in
2007 and 29 per cent in 2002 (when the contribution of natural change was smallest since
mid-1998). More information is given in the press release and statistical bulletin published on
27th August, available from:

Q2 Where can I find the figures?

Population estimates and key related products are available from:
Q3 How are the population estimates compiled?

Most ONS population estimates, including the national and local authority estimates are
calculated using the cohort component method. This is a standard demographic method in
which information on the components of population change are used to update a population
base such as the Census estimate. The resident population, by single year of age, on 30
June of the year prior to the reference year is aged on by one year; those born during the
12-month period prior to the mid-year are added on to the population and those who have
died are removed.

Other factors taken into account are the movement of people into and out of the UK
(international migration) and, for estimating the population for different areas within the UK,
movements between areas of the UK (internal migration).

Some population subgroups such as prisoners, school boarders and armed forces (home
and foreign) are estimated separately from the rest of the population. These groups are not
covered by the data sources used for migration estimates and therefore their movements are
not accounted for. In addition the age structure of these population subgroups tends to
remain constant over time so they are not aged forward.

Different methods are used to produce population estimates for some geographies, for
example National Parks and wards.

Full details of the methodologies used in producing population estimates can be found in the
documents available from:

Q4 Why don’t the mid-year estimates include short-term migrants?

Mid-year population estimates refer to the usually resident population of an area. Usual
residence is the standard United Nations definition for population estimates and includes
people who reside in the area for a period of at least 12 months. Only long-term
international migrants, that is a people who enter or leave the UK for a period of at least one
year are included in the estimates (see definitions in Box One below).
       Box one: United Nations definition of migrants
       Long-term international migrant
       The United Nations recommended definition of a long-term international
       migrant is:
            A person 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.

       This 12-month migrant definition is used for the UK usually resident population
       estimate series.

       Short-term international migrant
       The United Nations recommended definition of a short-term international
       migrant is:
            A person 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), except in cases where the movement to that country is for
            purposes of recreation, holiday, visits to friends and relatives, business,
            medical treatment or religious pilgrimage. For purposes of international
            migration statistics, the country of usual residence of short-term migrants
            is considered to be the country of destination during the period they
            spend in it.

       UK population estimates do not currently include short-term in-migrants as
       usually resident in the UK, nor do they exclude short-term out-migrants from
       the usually resident population.

ONS publishes separate estimates of short-term migrants to supplement the population
estimates. Mid-2007 estimates of short-term migrants for England and Wales were published
in February 2009. ONS plans to publish estimates of short-term migrants for local authorities
in England and Wales, for mid-2007, in October 2009.

Q5 What data sources are used to compile population estimates?

The most recent Census is used as the base for calculating the annual population estimates
between Census years.
Data for births and deaths, used within the cohort component method to update the Census
based estimates, are obtained from the General Register Office through compulsory
registrations of all births and deaths occurring in England and Wales.

Internal migration, that is the movement of people within England and Wales, is estimated
          The NHS Central Register (NHSCR) which records movements of patients
             between Health Areas; and
          Annual downloads of each patient register from Health Areas which is used to
             count the number of people who re-register or notify their GP of a change in
International migration is estimated from a number of sources including the:
            International Passenger Survey (IPS);
            Labour Force Survey data;
            Home Office data (for the number of asylum seekers and their dependents);
            Irish Quarterly National Household Survey and the National Health Service
             Central Register data (for estimates of flows between the UK and the Republic of
   A wide range of other data sources are used to estimate the number of emigrants at sub
   national level through a regression model.

Q6 Why don’t ONS use more administrative sources to produce
population estimates?

Administrative data are used in the production of some ONS population estimates, for
example the patient register data are used within the cohort component method to estimate
internal migration, and child benefit data are used in producing the National Park and ward
population estimates.

ONS is investigating whether other data sources can be used to improve estimates of
migration and/or resident population. The Ministerial Group and the senior Migration
Statistics Improvement Programme Board, both set up to oversee the improvement work
programme, have fully endorsed sharing data across government to improve migration
statistics. ONS is in the process of using the Statistics and Regulation Services Act 2007 to
establish legal gateways for information sharing between the UK Statistics Authority and
other public authorities and agencies. Data sources that are being investigated include
School Census data from the Department of Children, Schools and Families (DCSF),
Department of Work and Pensions (DWP) Migrant Scan data on National Insurance Number
(NINo) allocations to overseas nationals and Higher Education Statistics Authority data.

At the same time as establishing these legal gateways to data sharing, research is being
undertaken to assess how to make best use of these administrative data sources. This
research has identified some key issues that need to be addressed before these sources
could be used for the purpose of estimating statistics on the resident population. For
instance, four key issues are:

   1. Population coverage:
         a. ONS population estimates are based on the UN definition of long-term migrants
           (someone who changes their country of residence for a period of at least one
           year). Some registration data also include people who stay for less than one year
           and it is not possible to identify long-term migrants separately.
         b. Population estimates account for long-term migrants coming to or leaving the UK
           regardless of their age, nationality or reason for migrating. Registration data sets,
           cover the target populations that are relevant to the purposes of the administrative
           process, for example NINos allocations include overseas nationals who register to
           work or claim benefits.

    2.   Registration data such as Worker Registration Scheme (WRS) and NINo allocations
         reflect flows of migrants into the country but do not reflect out flows.

    3.   Some registration data refer to the date a migrant registers rather than the date they
         arrive in the country. Those migrants with existing NINos, such as returning British
         people, will not be recorded as a migrant in the NINo data set.

    4.   At a local level, published WRS figures relate to the address of the employer. For
         NINo flows the persons address relates to first registration which may differ from the
         area of first residence.
Q7 Are the population estimation methods robust?

The data sources used for the population estimates are the best that are available on a
nationally consistent basis. Information from administrative registers such as the numbers of
births and deaths is reliable. However, estimates of international migration are largely
derived from the International Passenger Survey (IPS) and are subject to sampling and non-
sampling error. National figures have relatively small levels of uncertainty, but at local level
the sample counts in the IPS are small and it is necessary to combine data across years
and/or local areas. International migration estimates at local authority level are made by
distributing higher level estimates using other data sources.

To account for internal migration, NHS patient registers are used as changes in address are
recorded. Although patient registers provide the best fit to the usually resident population,
they have recognised limitations with regard to timeliness and coverage.

The Statistical Quality Report for the Population estimates is available at:

ONS is leading a substantial cross-government programme of work to improve population
and migration statistics. The recommendations of the National Statistician’s
Interdepartmental Task Force on Migration Statistics (2006) are being implemented through
this work programme. The programme includes work to estimate and describe the quality of
population estimates.

Q8 Why is the number of births rising, and is this due to international

Recent increases in the number of births result from two factors:
       changes in the size and age structure of the female population;
       and changes in fertility rates.
The UK Total Fertility Rate (TFR), which can be interpreted as the number of children a
woman would have during her lifetime if she were to experience the fertility rates of the
period at each age, has risen from 1.63 in 2001 to 1.96 in 2008.

At the same time the total number of women in the key childbearing ages (15 to 44) living in
the UK has increased by 1.8 per cent between mid-2001 and mid-2008, meaning there are
more women who could potentially have a birth. Both these factors have led to the increase
in the overall number of births.

In England and Wales, between calendar years 2007 and 2008, the number of births to
women born in the UK increased by 1.5 per cent to 537,900 in 2008. Over the same period
the number of births to foreign born women increased by 6.5 per cent, to 170,900.

The rise in the number of births can be attributed to rising fertility rates among UK born
women. In contrast, the rise in the number of births to non-UK born women can be attributed
to the increase in the population of women born outside the UK, particularly at the ages
where fertility is highest.

Q9 Are migrants from accession countries included in population

The migration estimates (and population estimates) include citizens who move for at least 12
months from the eight central and eastern European countries which joined the EU in May
2004. For the 2008 mid-year period, immigration of citizens of the accession countries
remained stable at 102,000. Emigration by these citizens continued to grow from 27,000 in
the year to mid-2007 to 41,000 in the year to mid-2008.

Q10 What proportion of all international migrants were British and where
are British Citizens emigrating to?

Based on the (provisional) mid-2008 estimates of long-term international migration used in
the compilation of the mid-year estimates, 14 per cent of immigrants were British citizens.
This compares with 13 per cent for the year to mid-2007 and 16 per cent for the year to mid-
The mid-2008 figures for long-term emigrants showed that 45 percent were British citizens.
This compares with 51 per cent for the year to mid-2007. Estimates from the international
passenger survey show that for mid-2008, over half of British emigrants went to live in one of
five countries, Australia, Spain, Germany, USA and New Zealand.

Q11 What is ONS doing to improve the methods and data sources it

ONS has a continuous programme of review for its population and migration statistics and is
implementing a schedule of improvements to the data sources and methods used.
Developments implemented to date include:

                    An Increase in the sample size for international out-migrants in the
                     International Passenger Survey (implemented from January 2007).
                    New questions included in the IPS to improve estimates of how many
                     people change their original intention on length of stay when they enter
                     or leave the UK (implemented from 2004).
                    Improved methods for estimating the distribution of international
                     migrants for sub-national areas (implemented in 2007):

These improvements were incorporated in the first release of mid-2006 and mid-2007
population estimates and in revisions to the previously published estimates for mid-2002 to

A substantial work programme has subsequently been established to take forward the
recommendations of the 2006 Interdepartmental Task Force on Migration Statistics. This
work programme is being overseen and steered by a senior cross-government Migration
Statistics Improvement Programme Board and a Ministerial group. The Programme Board
agreed that further work plans will focus on:

                 Improving the data available on the numbers of people entering and
                  leaving the United Kingdom
                  Making effective use of new and existing administrative and survey data
                  Improving local population estimates and projections used in allocating
                   resources and developing services
                  Improving the public reporting of population migration statistics and
                  Establishing a wider range of timely indicators and analysis to inform the
                   evidence base on migration and its impact on policy and public services.

Five interdepartmental working groups have been established to cover each of these
elements of work. Further details and updates on the progress of this work are available at:

Q12 When will ONS make further improvements

A full package of improvements to be implemented in the short term was announced in
February 2009. In summary, this package incorporates:
• The use of administrative sources instead of 2001 Census estimates for the distribution of
international migration to the local authority level.
• Improved methods for estimates of moves within the country (internal migration) that take
account of administrative data collected by the Higher Education Statistics Agency on
students term-time location.
• Refinements to some of the methods previously introduced in 2007.
• Estimates of short-term migration at a more detailed geographic level (local authority).
• Production of provisional international migration estimates to improve timeliness and
provide an earlier indication of migration at the national level.
• A suite of migration indicators at the local authority level.

ONS will publish revised 2002-2008 mid-year population estimates in May 2010 which reflect
these improvements. The new methods will be used for the first release of the 2009 mid-year
population estimates in summer 2010.

Further details of these improvements and the longer term programme are available in the
“Migration Statistics Improvement Work Programme (IMPS) Updates”, at: