Background Information and Methodology
Background Information and Methodology
Migration is the most difficult component of population change to estimate; there is no comprehensive system which registers migration flows in and out of the UK – whether migration to or from overseas (international migration), migration to or from other parts of the UK or migration within Scotland. Estimates of migration flows therefore have to be based on the best proxy data that exist. The text below outlines the data sources and methods used to estimate migration flows available on this website.
Methodology used to produce National Records of Scotland (NRS) official migration flows
1. Sources of migration data
In 2021, the key sources of migration data were:
- The National Health Service Central Register (NHSCR) is used to calculate moves between NHS Board areas within the UK. It is also used to distribute estimates of international migration to NHS Board areas in Scotland.
- Community Health Index (CHI) is used to estimate migration at Council area and below.
- Other administrative data provide information on moves into and out of Scotland with an origin or destination of outside the UK.
The use of these sources in producing internal (within UK) and international migration estimates is described below.
2. Internal migration
The NHSCR system records the movements of patients between NHS Board areas in Scotland, whereas the movements for patients in England and Wales are recorded in the Personal Demographic Service (PDS). The PDS holds the master demographics database for the NHS in England and Wales. Each time a patient transfers to a new NHS doctor in a different NHS Board area, the NHSCR and PDS are notified and then the patient is considered to have made a migrant move. Counts of these re-registrations are used as a proxy indicator for moves between Scotland and the rest of the UK.
The CHI holds records of people registered with an NHS doctor in Scotland. The records provided to NRS contain the postcode of the patient's address, which enables migration to be estimated for councils, and for smaller areas.
Currently, NRS 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.
For mid-2015 onwards an improved method for estimating internal migration within the UK has been introduced. We are now using a direct extract of anonymised records from the NHSCR to calculate the migration at health board level. More information can be found in the Methodology Guide for the mid-year population estimates for Scotland available on this website.
3. International migration
An international migrant is defined by the United Nations as someone who changes country of residence for 12 months or more. There is no single system in place to measure all movements of people into and out of the UK or to determine if they meet the definition of a long-term migrant. 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 a useful source of data, none of the data sources used are specifically designed to capture information solely on international migration. In recent years, methods to produce international migration have changed. The different methods are detailed below.
3.1 International migration: Pre mid-2020 estimates
Prior to mid-2020, NRS used the Long-Term International Migration (LTIM) estimates produced by the Office for National Statistics (ONS) for the national estimates of international migration to and from Scotland.
Three sources of data were used by ONS to compile the LTIM estimates:
- International Passenger Survey (IPS);
- Information held by the Home Office; and
- Labour Force Survey (LFS).
The IPS is a continuous sample survey conducted by ONS at the principal air, sea and Channel Tunnel routes between the UK and countries outside the British Isles. It has been 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. The Home Office provides data on refugees and on asylum seekers and their dependants, and the LFS collects information on where international migrants live based on their recorded work address.
Over time changes have been made to the IPS sample design, 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 which means that the IPS now includes Edinburgh, Glasgow and Aberdeen airports, and improvements to the IPS weighting methodology.
Given that the IPS collects information on the intentions of potential migrants, which may or may not be realised, an adjustment is made to the IPS data for visitor switchers (people who say they are staying for less than 12 months but in fact stay more) and migrant switchers (people who say they are staying for longer than 12 months but in fact stay for fewer).
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 including the quality of the IPS can be found in the International Migration Methodology section of the ONS website.
3.2 Mid-2020 estimates: Impact of COVID-19
In March 2020, the IPS was suspended due to the COVID-19 pandemic. The Office for National Statistics led research into alternative data sources including using statistical modelling to estimate UK international migration over this period. For the year ending June 2020, international migration for Scotland was estimated by the following:
- July 2019 to February 2020: Estimate migration flows based on the established method using IPS data. Migrant and visitor switcher data was only included for July to December 2019.
- March to June 2020: Migration flows estimated based on the Scottish proportion of UK modelled migration. The Scottish proportion of the modelled migration was based on the Scottish proportion of IPS flows from July 2019 to March 2020.
- Year to June 2020: Include data on asylum seekers and refugees for the full year.
3.3 Mid-2021 estimates
It has long been acknowledged that the IPS, which underpins previous estimates of international migration, has been stretched beyond its original purpose. In response to the IPS being suspended due to COVID-19, ONS accelerated their approach for transforming migration statistics. The latest figures for mid-2021 are therefore produced using a new method that relies less on IPS data and statistical modelling, and makes greater use of administrative data. Because of this change in method, the latest figures on international migration may not be comparable with previous estimates.
The latest estimates use different data sources and methods for each nationality grouping:
- Non-EU nationals: Estimates are based on the Home Office initial status analysis (ISA) system, which combines visa and travel information to link an individual’s travel movements into and out of the country. This dataset is known as the exit checks dataset.
- EU nationals: Due to free movement between the EU and the UK during the reference period, visa data cannot be used for EU nationals. Instead, the methodology to estimate the migration of EU nationals is based on previous research to measure international migration using the Registration and Population Interaction Database (RAPID).
- British nationals: IPS data are still the main source of information. The IPS was reinstated in January 2021 and was therefore used for estimates for January to June 2021. Modelled data has been used to cover the period when the IPS was suspended (March to December 2020), with the assumption that the pattern of British nationals’ immigration to the UK is equivalent to non-EU nationals’ emigration from the UK, and vice versa.
Going forward, the revision of long-term international migration statistics will be an important part of the production of these estimates. Provisional estimates are released with the expectation they may be revised as more complete data become available. In addition, methods are still experimental and estimates will therefore be revised as the new methods mature.
The latest provisional UK-level long-term international migration figures for mid-2021 can be found on the ONS website. More information about how the latest figures are produced can also be accessed on the ONS website.
3.4 Distribution of International Migrants within Scotland
International migration flows between Scotland and any country outside of the UK are allocated to each NHS Board area and given an assumed age and sex distribution using proportions recorded on the NHS Central Register (NHSCR). These proportions are applied to the total international migration flows for Scotland.
For inflows the recorded moves from outside the UK to Scottish NHS Board areas on the NHSCR are used to estimate the proportion of migrants entering each area.
NHSCR records hold limited data on international outflows from NHS Board areas to countries outside the UK, as patients do not reliably de-register with their doctor when moving internationally. As a result of this, various methods have been used to approximate the geographic distribution of out-migrants since 2001. We recently reviewed the current method and found that it still provides the best approach for estimating the age and sex distribution of international outflows from each NHS Board area given data sources available.
International outflows are allocated to NHS Board areas based on:
- international inflows from two years prior to the reference date;
- outflows to the rest of the UK; and
- the population share of each NHS Board area.
Age and sex distributions of international migrants are obtained from the NHSCR. For in-migrants, the age / sex distribution of moves from outside the UK to the NHS Board area on the NHSCR is used. For international out-migrants the age / sex distribution is based on out-migrants to the rest of the UK leaving from the NHS Board area, and in-migrants from outside the UK two years prior to the reference date, aged on by two years.
It is acknowledged that NHSCR flows undercount the number of migratory moves for young men in particular, due to General Practitioner (GP) registration behaviour in different groups. Compared with LTIM estimates by sex there are fewer men in both the in- and out-migrant groups recorded on the NHSCR. A sex-ratio adjustment has been introduced from 2011 which increases the number of male migrants at young adult ages where there is a large majority of women seen in the NHSCR data. More information can be found in the Methodology Guide for the mid-year population estimates for Scotland available on this website.
Distribution of migrants to council areas is based on the Community Health Index (CHI) records, controlled to the NHSCR geographic and age / sex distributions at NHS Board area level. International in-migrants were allocated using records appearing on the CHI extract where the patient had arrived from outside the UK. Like the NHSCR, the CHI extract holds limited data on people leaving Scotland for any country outside the UK and so international out-migrants were allocated using a combination of international in-migrants to Scotland and migrants leaving Scotland for the rest of the UK.
Quality assurance arrangements for administrative data
Further information on the quality assurance arrangements for administrative data used in migration estimates can be found in the National and Official Statistics section of the website.