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Mid-2011 Population Estimates Scotland

5. Notes and definitions

Population covered

The estimated population of an area includes all those usually resident there, whatever their nationality. Students are treated as being resident at their term-time address. Members of UK and non-UK armed forces stationed in Scotland are included; UK forces stationed outside Scotland are excluded. Short-term international migrants, who stay for less than 12 months, are excluded.

Age

Population figures relate to 30 June of the year shown and ages relate to age at last birthday.

Presentation of estimates

Although the populations are tabulated in units, this does not imply accuracy to that level. The data are presented in units for the convenience of users wishing to compile non-standard aggregations without encountering rounding problems.

Administrative areas

The composition of the NHS Board areas in terms of Council areas is summarised in the table below. From 1 April 2006 responsibility for NHS Argyll & Clyde was split between NHS Greater Glasgow (now known as Greater Glasgow & Clyde) and NHS Highland. The tables presented in this report show information for the new NHS Board areas for all years.

Composition of NHS Board areas

1 New NHS Board areas including parts of former Argyll & Clyde.

Note: In practice there are some other very small 'slivers' where NHS Board and Council area boundaries cross.

Land area

The land areas used to calculate the population density information presented in Table 9 were derived from digital boundaries used for the 2001 Census.

Methodology and comparisons across the UK

The National Records of Scotland (NRS) website has a guide that describes the methodology used to produce the mid-year population estimates for Scotland. This methodology is similar to that used to produce population estimates in other UK countries. A comparison of data sources and methods used for population estimates across the UK is available from the Office for National Statistics (ONS) website.

Sources of migration data

Migration is the most difficult component of population change to estimate, as there is no comprehensive system which registers migration in the UK, either moves to or from the rest of the world, or moves within the UK. Estimates of migration have therefore to be based on survey data and the best proxy data that exist.

Migration estimates are derived from the following key sources of data. The National Health Service Central Register (NHSCR) is used to calculate moves between NHS Board areas within the UK, with migration at Council area and below estimated using anonymised data from the Community Health Index (CHI) supplied with the permission of the Scottish Directors of Public Health. The International Passenger Survey (IPS) provides information on moves into and out of Scotland with an origin or destination of outside the UK. In addition to IPS data, additional information is used for asylum seekers. These three main sources are described in more detail in the remainder of this section.

More information on the methodology used to estimate migration can be found in the migration section of the NRS website and in the Mid-Year Population Estimates for Scotland Methodology Guide.

Internal migration within the UK

The NHSCR system records the movements of patients between NHS Board areas in the UK. Each time a patient transfers to a new NHS doctor in a different NHS Board area, the NHSCR is notified and the patient is then considered to have made a migrant move. Counts of these re-registrations are used as a proxy indicator for moves within the UK.

The CHI holds records of people registered with an NHS doctor in Scotland. The records used in production of the mid-year population estimates contain the postcode of the patient's address, which enables migration to be estimated for Council areas, and for smaller areas. The approach used for estimating Council area-level migration involves matching CHI patient records extracted from a database which reflects the 'live' CHI system on two occasions one year apart.

Currently, migration data derived from the NHSCR is considered to be the most reliable data available at NHS Board level, so estimates from the CHI are controlled to ensure that they are consistent with the NHSCR data for moves across a NHS Board boundary by origin, destination, age and sex.

Due to an improvement in data sources, the accuracy of cross border migration data between Scotland and Northern Ireland was improved from mid-2009. More information on the nature of the improvements is contained in the papers from the May 2010 meeting of the Population and Migration Statistics Committee (PAMS), available on the NRS website. We are reviewing the process for estimating internal migration, including investigating the use of patients' postcode information held on the NHSCR.

International migration

An international migrant is defined by the United Nations (UN) as someone who changes country of residence for 12 months or more. There is no single, all-inclusive system in place to measure all movements of population into and out of the UK. Therefore, it is necessary to use a combination of data from different sources that have different characteristics and attributes in order to produce estimates of international migration. While offering the best data currently available, none of the data sources used is specifically designed to capture information solely on international migration.

Three sources of data are used to compile the national estimates of international migration into Scotland:

• The International Passenger Survey (IPS) is a continuous sample survey conducted by ONS at the principal air, sea and Channel Tunnel routes between the United Kingdom and countries outside the British Isles. It is the prime source of migration data to and from the UK, providing estimates of both inflows and outflows, but does not cover all migration types;
• Home Office (HO), which provides data on asylum seekers and their dependants;
• The Labour Force Survey (LFS), which collects information on where international migrants live.

Fundamental changes were introduced to the IPS sample design at the beginning of 2009 to make the survey more focused on identifying migrants. These changes included a re-organisation of the times and frequency of sampling of travellers, sampling at additional locations including Aberdeen and improvements to the IPS weighting methodology.

Given that the IPS collects information on intentions which may or may not be realised, an adjustment is made to the IPS data for visitor and migrant switchers (people who change their intentions and their migratory status). The Long-Term International Migration (LTIM) estimate (previously referred to as Total International Migration, TIM) is derived by combining these components.

The IPS provides an estimate of international migrants into the United Kingdom. The allocation of these migrants to Scotland is based on the LFS. The LFS provides more reliable data on the geographical distribution of immigrants than the destination provided in the IPS as it is based on where migrants actually live rather than on their initial intentions. The outflow of international migrants from Scotland is based directly on the IPS data. More information on the LTIM method can be found on the ONS website.

The LTIM estimate for Scotland has been used in the mid-year estimates for Scotland since mid 2007. Previously, a similar method based on the IPS was used, but this did not use the LFS distribution or include estimates of migrant switchers. It did however use an adjustment for unmeasured migration. The results of the 2001 Census indicated that the previously published 2000 mid-year estimates had been overestimated as a result of cumulative errors in estimating migration. To ensure that migration estimates did not continue to be overestimated, an unmeasured migration adjustment was included. The LTIM methodology does not include this adjustment. Rather the assumption is that, since the census and estimates are calculated using different methodologies, differences inevitably arise. An evaluation of the impact of changing method has been carried out. The conclusion was that the previous method estimated a cumulative net migration gain around 9,700 higher than the TIM method between the census and the 2006 estimates. As this is not considered substantial, there are currently no plans to revise the previous mid-year estimates due to this change.

Distribution of international migrants to areas within Scotland

International migration flows to NHS Boards were allocated by using overseas inflows recorded on the NHSCR. However, NHSCR records hold limited data on international outflows from NHS Boards to overseas, as patients rarely de-register with their doctor when moving overseas. As a result international outflows were allocated using averaged proportions based on international inflows, outflows to the rest of the UK and the population size of each NHS Board.

Age and sex distributions of international in-migrants are obtained directly from the NHSCR. The age/sex distribution of moves for international out-migrants was based on the distribution of migrants to the rest of the UK, as recorded by the NHSCR.

The distribution of migrants to Council areas is based on records from the Community Health Index (CHI), which are made consistent with the NHSCR geographic and age/sex distributions. International in-migrants were allocated using records appearing on the CHI extract where the patient had arrived from overseas. Like the NHSCR, the CHI extract holds limited data on people leaving Scotland for overseas and so international out-migrants were allocated using a combination of in-migrants to Scotland from overseas and migrants leaving Scotland for the rest of the UK.

Work to improve the method used to allocate international out-migrants to NHS Board and Council areas is continuing and further improvements will be considered for future publications.

Additional migration data

The NRS website includes a migration section which provides additional tables showing migration estimates associated with mid-year population estimates.

Research into improving migration and population statistics

Since the early 2000s, and especially since Eastern European countries joined the EU in May 2004, migration has played a larger part in Scotland's demographic change than in the previous decade. So it has become more important to have high quality statistics on migration and the population, for policy development and for planning and providing public services. NRS was part of an inter-departmental effort, led by the Office for National Statistics (ONS), to improve the estimates of migration and migrant populations in the United Kingdom, both nationally and at a local level. The ONS website has more information on the Migration Statistics Improvement Programme including the programme final report.

Although the programme has now finished, work to improve migration statistics is continuing. In Scotland the key focus will be reconciling the mid-year estimates with the 2011 Census results and rebasing historical estimates. As part of this work we will review our method for estimating migration.

Publication of future population estimates following the 2011 Census
We plan to use the 2011 Census to revise the mid-2011 population estimates and the estimates for previous years. The timings of the publication of the revised estimates and the estimates for mid-2012 have yet to be finalised.

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