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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.
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.
The NHSCR system records the movements of patients between NHS health board areas in the UK. Each time a patient transfers to a new NHS doctor in a different health 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. Unlike the NHSCR, the records provided to the National Records of Scotland (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 health board level, so estimates from the CHI are controlled to ensure that they are consistent with the NHSCR data for moves across a health board boundary by origin, destination, age and sex.
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 Home Office and the Irish Central Statistics Office.
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 Total International Migration 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 Labour Force Survey (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.
The TIM estimate for Scotland was used for the first time in the 2007 mid-year estimates for Scotland. The previous method was based on the IPS but did not use the LFS distribution or adjustments for migrant switchers. However, it did include an adjustment for unmeasured migration.
International migration flows to NHS Boards are allocated using overseas inflows recorded on the NHSCR. However, NHSCR records hold limited data on outflows from NHS Boards to overseas as patients rarely de-register with their doctor when moving overseas. For the mid-2007 estimates international outflows were therefore allocated using averaged proportions based on international inflows, outflows to the rest of the UK and the population size of each Health Board. This is an improvement on the method used prior to 2007 in which the limited NHSCR data on international outflows was used in combination with moves from Scotland to the rest of the UK.
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.
Distribution of migrants to Council areas is based on the Community Health Index (CHI) records, controlled to the NHSCR geographic & 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.
Page last updated: 21 May 2012
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