Migration - Methodology
Migration - Methodology
Migration is the most difficult component of population change to estimate; there is no comprehensive system which registers migration in the UK – whether migration to or from overseas, migration to or from other parts of the UK or migration within Scotland. Estimates of migration 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 the migration estimates available on this website.
Methodology used to produce National Records of Scotland (NRS) official migration estimates
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 the UK. Each time a patient transfers to a new NHS doctor in a different NHS Board area, the NHSCR is 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 within 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 NHS Central Register (NHSCR) to calculate the migration at a 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 (UN) as someone who changes their country of residence for 12 months or more. There is not a single, all-inclusive system in place to measure all movements of population into and out of the UK. Therefore, a combination of data from three different sources is used to compile the National Statistics estimates of international migration into Scotland: the International Passenger Survey (IPS), the Labour Force Survey (LFS) and the Home Office (HO).
The IPS is a continuous sample survey conducted at the principal international air, sea and Channel Tunnel routes in the United Kingdom and provides an estimate of international migrants into the United Kingdom. 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 adjusted IPS data is called the Long Term International Migration (LTIM) (previously referred to as Total International Migration (TIM)) estimate and is produced by the Office for National Statistics (ONS). The allocation of Scotland’s share of international migrants to the UK is based on the LFS. The LFS provides more reliable data on the actual geographical distribution of immigrants, rather than the intended destination provided by the IPS. The outflow of international migrants from Scotland is based directly on the IPS data.
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 re-organisation of the times and frequency of sampling travellers, sampling at additional locations including Aberdeen and improvements to the IPS weighting methodology. As a result of these methodological changes, the Long Term International Migration back series was revised.
The international migration estimates presented here are the published LTIM estimates except for the years 2002 to 2011. The international migration estimates for these years have been revised to take into account the results of the 2011 Census. Therefore they will differ to the LTIM estimates published by ONS for those years. More information on the revision can be found on this website.
3.1 Distribution of International Migrants within Scotland
International migration flows to NHS Boards areas are allocated using overseas inflows recorded on the NHSCR. However, NHSCR records hold limited data on outflows from NHS Board areas to overseas as patients rarely de-register with their doctor when moving overseas. International outflows are currently allocated to NHS Board areas based on flows of other groups of migrants which stand as a proxy for overseas out-migrants.
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 improved for mid-2012 and is now 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. It is acknowledged that the NHSCR flows undercount the number of migratory moves, for men in particular, due to GP registration behaviour in different groups. A sex-ratio adjustment has been introduced 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. International in-migrants were allocated using records on the CHI extract where the patient had arrived from overseas. 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.