ONS is responsible for producing estimates of internal migration in England and Wales. However, measuring migration is not straightforward, as there is no compulsory system within the UK to record the movement of the population. Therefore, it is necessary to use proxy data to make estimates of migration.

Migration estimates are based on patients moving and changing their doctor as they change address. The NHS Central Register (NHSCR) at Southport records movements of patients between Health Authority areas (HAs). Historically internal migration estimates were available at the Family Health Service Authority (FHSA) level; these were equivalent to shire counties, metropolitan districts and groupings of London boroughs. In 1996, FHSAs in England and Wales were replaced by HAs. FHSA-level migration estimates are available from 1975 on both a quarterly and annual basis, and are published in Population Trends, Key Population and Vital Statistics and other ONS publications such as Social Trends and Regional Trends. However, until recently, no migration estimates were published below FHSA/HA level.

In 2000, internal migration estimates by age and gender were made available for all Local Authorities (LAs) and Health Authorities (HAs). These estimates are made by combining the migration estimates at the HA level based on NHSCR data, with information from the patient registers held by the individual health authorities. Mid-year data was first produced for 1998 and has been made available for each year since then. The latest data is now available for mid 2001. Summary data will be published in Population Trends and made available on the National Statistics Website. Further data are also available from ONS on request (see Section 5).

The NHSCR began life as the National Register, which was used to issue identity cards and ration books and to help with the call-up for the Armed Forces in WW2. When the NHS was set up in 1948, the National Registration numbers were used to ensure that each patient’s record had a unique identification. After rationing ended, the register was retained as the Central Register for the NHS for England and Wales. This ensured that FHSAs maintained up to date lists of patients resident in their areas. Today, the NHSCR at Southport provides a comprehensive system to assist with NHS patient administration in England and Wales.

The work of the NHSCR covers a range of services; one of these is to record the transfer of patients between HAs. These data are collected and used as a proxy for internal migration estimates. Each record in the register contains the NHS number, name, date of birth and date of acceptance by the HA. The entries on the register are updated on receipt of information from HAs. If the updating involves a change of HA, (because the patient’s new GP falls within a different HA) a ‘migration’ record is created. In addition to patient moves, migration records are created when patients remove themselves from NHS doctor’s lists to enter the armed forces, and then again when they return to civilian NHS doctors. Finally, migration records are created for those emigrating and later returning, and for new immigrants from outside the UK. However, some patient moves do not count as migrations. For example, moves to long-stay psychiatric hospitals and imprisonment. In addition, internal migration estimates do not include the movements of armed forces.

When a NHS patient moves to a new HA and changes their GP, this change is recorded by the NHSCR. However, the NHSCR does not record information on the actual change of address. For the purpose of making migration estimates, it is assumed that a change of doctor to a new HA represents a change of address to a new HA area. It is also assumed that the average delay between moving house and registering with a new GP is about one month.

The NHSCR supplies ONS with weekly data. The data contains records of all the between-HA moves that have been registered in the last week. The data are processed and tabulated, then supplied to customers each quarter on a rolling year basis, as migration estimates between HAs.

The accuracy of the data depends on all patients re-registering with a doctor when they move. However, it is known that re-registration patterns vary by sex and age group. For example, young children, their mothers and the elderly usually re-register quite quickly after moving, while young men take longer to re-register than women of the same age. In addition, some students register at their term time address while others remain registered at their parents’ address.

The NHSCR system can only record moves between HAs. Therefore, migration at lower geographical levels cannot be estimated using the NHSCR data. This is a major limitation, as there is demand for migration estimates between areas smaller than HAs. Until recently, in compiling population estimates, ONS has used information from electoral registers and the most recent census to apportion the NHSCR inflows and outflows between the constituent local authorities. A limitation of using electoral registers to estimate migration is that there are sometimes variations in the registers caused by the procedures used to maintain them. In addition, people (especially students) may be registered in more than one place. Age and gender is not recorded on the electoral register, so an age and gender distribution derived from the preceding census has been applied.

The limitations of the data resulted in the investigation of alternative data sources, to see whether they could be used to produce estimates of greater quality, or for smaller geographical areas. This led ONS to investigate the patient registration data held by HAs themselves as a possible source. This data source is referred to in these notes as ‘Patient Register data’.

Like the former FHSAs, HAs hold records of patients who are registered with a NHS GP in England and Wales. The relationship betweenHAs and the NHSCR is shown in the diagram below, which illustrates the flow of information between the NHSCR and the HAs in England and Wales.

The use of patient register data presented an opportunity to improve the quality of internal migration statistics and hence the quality of population estimates. ONS evaluated the patient registers as a source of migration estimates and concluded that:

1. Data from the patient registers could be used to provide migration estimates that are consistent and plausible over time.
2. The data could be used to provide annual estimates of migration down to LA and HA level.
3. The patient registers were expected to be significantly better than the electoral rolls for estimating movement of the population.
4. The quality of the information held on patient registers had improved over time and was expected to continue to improve.
5. However, the data needed to be used in combination with the existing NHSCR data; it could not be used directly because of certain limitations that should be addressed so that the best use could be made of the data.

The limitations in using migration estimates derived solely from the patient registers are:

A small number of records had a missing data item. This was particularly important if the postcode was missing. The missing data item has been completed using the information held on other patient records.

In addition, by comparing patient registers in two consecutive years, certain groups of moves that occur during the year are missed. This is because patient registers cannot capture the movement of those migrants, who for one reason or another were not registered with a doctor in one of the two years, but who moved during the year. The largest group of these is migrant babies aged less than one year, who would not be on a register at the start of the year. Other people, who are not on the register at the start of the year but who move after joining the NHS and before the end of the year, would not be captured. Such people could include those leaving the armed forces or international in-migrants. Similarly, people who move within the year but are not on a register at the end of the year would also not be captured. Such people would include anyone who moved and then, before the end of the year, either died or enlisted in the armed forces or left the country. To address this, the more complete information from the NHSCR has been combined with the more geographically detailed data from the patient registers, to produce migration estimates for LA and HA areas.

Every HA (formerly FHSA) holds a register of the patients registered with GPs within their area of responsibility. This contains the NHS number, gender, date of birth, date of acceptance at the health authority and importantly the postcode of address, for each patient. By obtaining a download from each patient register on an annual basis and by combining all patient register extracts together, ONS can create a total register for the whole of England and Wales. Comparing records in one year with those of the previous year by linking on NHS number enables identification of people who change their postcode. A migrant is defined as a person who, between one year and the next, changes their area of residence. The download is taken at 31 July each year to enable migration estimates to be made for the year ending 30 June each year. This is consistent with the assumption that people delay registering with a new GP for a month after they move. This process comprises the following steps:

1. Duplicate records are removed (those with the same NHS number).
2. Temporary records are removed (patients who are temporarily registered somewhere to receive treatment but do not count it as their usual place of residence).
3. Using the postcode information, a series of area codes are added to the records to identify the LA and HA areas in which people on the register live.
4. Combining patient register extracts together gives a total register for the whole of England and Wales.
5. Comparing records in one year with those of the previous year by linking on NHS number enables identification of people who change their postcode. A migrant is defined as a person who, between one year and the next, changes their area of residence.
6. The patient register data are combined with information from the NHSCR data to produce internal migration estimates.
7. Inflows and outflows are tabulated by age and gender for the geographical areas of interest.

While using patient registers alone does not capture all migrant moves, combining NHSCR data with patient register data to produce migration estimates gives rise to several potential advantages. In the patient registers, information is held on the residential address of the patient i.e. the postcode. Therefore, up-to-date migration estimates can be produced for LAs and HAs by age and gender. In addition, as these estimates are derived by aggregating postcodes, migration estimates from the registers can be produced for any geographic level above postcode, (however, at this stage, migration estimates are not available for areas smaller than LAs). Migration estimates for different boundaries can be created simply by changing the input postcode/area files. For example, to produce counts of people who moved in 1996-1997 between areas that existed in 1995, the 1997 postcode/1997 area file is replaced with a file containing 1997 postcodes and 1995 areas.

Until now, the Census has been the only source of detailed statistics for migration between relatively small regions of England and Wales (i.e. by age and gender, and for areas such as wards and local authority districts). Unlike the Census, extracts from the patient registers are available for a number of years. This means that it is possible to use more up-to-date information to estimate migration for LAs and HAs by age and gender.

It is mentioned above that patient register data are combined with NHSCR data to produce estimates of migration. This means that the patient register system essentially allows the existing HA level migration estimates to be disaggregated to local authority districts. The two systems are complementary; the patient register data does not replace the NHSCR data.

Additionally, although using patient registers gives ONS the capability to produce migration estimates at very small geographic levels, confidentiality issues are likely to prevent the publication of such estimates.

There is one limitation that is common to both data sources; the data appear to under record the movements of males aged 16-29, compared to the movement of females of a similar age. However, there is insufficient information about the extent of this undercount to allow accurate adjustments to be estimated at HA and LA levels. A possible method was identified but further work is needed on two counts to develop this methodology before it can be applied - any adjustments that are available must be area specific, and the cause of the male undercount must be ascertained. Future data from the Labour Force Survey, which will be collected for a larger sample, and data from the 2001 Census can be used to investigate this further.

There are a number of differences between the published migration estimates from the two data sources:

• NHSCR data are collected weekly then processed and delivered quarterly on a rolling year basis. Collecting and processing the patient register data, on the other hand, is more time-consuming, and is carried out annually, from a mid-year download of data. This means that for three quarters of the year, only the estimates for HA areas are available, but once a year an extra set of tables are provided, showing migration estimates for areas below HA level.

• Age is defined differently for the two datasets. The quarterly estimates available from the NHSCR at the ha level show age at the time of migration. The migration estimates derived from the patient registers, while constrained to the totals of the HA level estimates from the NHSCR, represent age at the end of the mid-year period, not age at migration. This is because these data are used directly in the compilation of mid-year population estimates, and to estimate a rolled forward population estimate, age at the end of the mid-year period is required.

Because of this difference in age definition, people who migrated when they were 25 might well be 26 by the time of the annual patient register data download. Therefore, generally, a migrant will appear to be up to a year older on the patient register data than on the NHSCR data.

Internal migration data have been disseminated via Population Trends, Social Trends & Social Focus, Regional Trends and Key Population and Vital Statistics. The patient register data was introduced in 2000 (for years ending mid-1998 and mid-1999 tables) to complement the existing internal migration data.

The following migration outputs are available from ONS for twelve-month periods ending March, June, September and December. The earliest is that ending December 1975. The latest is usually ready about five months after the end of the period. These tables are based only on NHSCR data and do not include the patient register data.

LA estimates derived from the combination of NHSCR and patient register data are available annually from the year ending mid-1998 to the most recent year available. The tables below can be obtained free of charge from the National Statistics Website (www.statistics.gov.uk)

In addition, ONS have used the patient register data in the production of annual mid-year versions of the NHSCR-based origin/destination tables (tables 2a, 2b and 3). The following tables are available on request for the years ending mid-1999, mid-2000 and mid-2001:

Table 2a Origin/destination matrix of flows between LAs, or between HAs in England and Wales.

In addition to these standard outputs, the following types of data are available on an ad hoc basis.

A small charge will be made to cover the costs of production and distribution of origin-destination and

ad hoc tables.

Data are only available down to LA and HA levels. This is because the methodology to address the limitations has not been applied at other geographical levels. In addition, migration estimates for smaller geographical areas may be subject to confidentiality constraints.

While the outputs described above represent an improvement on the internal migration data available, customers have specified that they want to be able to track moves between smaller areas. However, this will only be possible once the data processing system has been redeveloped, and once the quality of the data at these low levels has been established. At that point, ONS will decide upon the output that should be made available. It could include:

• Moves from or to wards and/or origin/destination matrices for wards
• Moves from or to custom geographies
• Moves within LAs
• The capacity to provide specialist data to research agencies so that they can undertake projects of interest to, but outwith the remit of ONS.

To order internal migration tables, or for general enquiries, please email the ONS Migration Statistics Unit at migstatsunit@ONS.gov.uk or write to:

Migration Statistics Unit

Room 2300

Office for National Statistics

Segensworth Road

Titchfield

Fareham

Hampshire

PO15 5RR

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