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, migration to or from other parts of the UK or migration within Scotland. Estimates of migration flows therefore have to be based on survey data and 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
There are three key sources of migration data:
- 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.
- International Passenger Survey (IPS) provides 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.
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 are 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 less).
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.
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, overseas 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.
3.1 Distribution of International Migrants within Scotland
International migration flows between Scotland and overseas 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 overseas migration flows for Scotland.
For inflows the recorded moves from overseas 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 overseas, as patients do not reliably de-register with their doctor when moving overseas. 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 overseas to that 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 that NHS Board area, and in-migrants from overseas 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 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.
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.