- The method currently used to distribute international immigrant flows to local authorities cannot be improved by accounting for new insights into lagging in administrative data for migrants.
- Around 10,000 NHS Patient Register records used to distribute the flow of non-British 17- to 59-year-old, non-working, non-student immigrants (referred to as “other migrants”) in mid-2018 could be found as workers or students in previous years.
- Removing these records from the distribution of other migrants has no significant impact on the resulting international immigration estimates at local authority level.
- No changes will be made to the method used to distribute international immigrants to local authorities.
The mid-year population estimates for England and Wales use a mixture of survey, administrative and census data to distribute national international immigration flows to local authority level. For all processing since 2011, the principle has been to take the national immigration flows (long-term international migration), broken down by reason for migration, and use the most appropriate administrative data to distribute the flow to local authorities. The three main administrative data sources used in this process are:
- the Migrant Worker Scan (MWS) (for working migrants)
- data on higher education from the Higher Education Statistics Agency (HESA)
- the GP NHS Patient Register (for other migrants)
These three data sources are available to the Office for National Statistics (ONS) at a record level, and work is carried out to link them together so that duplication across the data used to distribute workers, students and other migrants can be addressed.
In January 2019, we published research into new international immigrants’ patterns of registration for a National Insurance number (NINo) and the NHS. Analysis showed new international immigrants do not register themselves for a NINo and with doctors surgeries at the same time. The lag between registrations could mean they appear in different reporting periods on different datasets. Given our use of linked administrative data sources, this means we could potentially include the same migration event, via different administrative data sources, in two successive years of our immigration distributions.
We have conducted research to assess whether lags in registering for administrative data sources impact our existing method for distributing international immigrants to local authorities. This research showed that we could link 10,000 (out of 180,000) of the GP patient registrations with a Flag 4 to MWS records from previous years. However, removing these records has a negligible impact on the resulting estimates of local authority international immigration as they are spread relatively evenly across England and Wales.Back to table of contents
The research we published in January 2019 showed clear differences between when new international immigrants appeared on administrative data sources. These included:
- EU nationals register more quickly for a National Insurance number (NINo) than non-EU nationals
- non-EU nationals register more quickly with the NHS than EU nationals (EU median lags: 276 days; non-EU median lags: 60 days)
- females tend to register more quickly with the NHS than males, with this trend seen in all except the youngest age group (males’ median lags: 291 days; females’ median lags: 150 days)
These findings imply a strong likelihood a proportion of migrants appearing as Flag 4s in one year may appear as worker or student migrants in other years. In effect, the same migration event could be informing multiple years of our immigration distribution, potentially leading to bias in our estimates.
However, because of the way our processing accounts for immigrants coming for work and study, it is highly unlikely that any migration events for these would appear in the other migrant stream the year before. This is because working migrants are only included in our distributions if their arrival date in the UK was within the reference period. Consequently, if a migrant did appear on the previous year’s NHS Patient Register as a Flag 4 and subsequently applied for a NINo in this year’s reference period, they would be excluded from our distribution of workers because of their arrival date. For Flag 4s, we only receive the date they registered with a GP rather than a self-reported arrival date.Back to table of contents
In carrying out the linkage work, we verified that international immigrants can appear in different administrative data sources in successive years and that the same migration event can influence two years of our immigration distributions. However, this work also shows that removing these records from the most recent year’s migration data has a negligible impact on the resulting distribution of international migrants that we use in the construction of population estimates.
Table 2 shows that around 10,000 of the Flag 4s used to distribute other migrants for mid-2018 linked to either a worker or student record for either mid-2017 (T minus 1), mid-2016 (T minus 2) or mid-2015 (T minus 3). At a high level, this research confirmed the findings of the January 2019 research with the majority of those having relatively short lags between their appearance on different administrative data sources. In our linkage of mid-2018 Flag 4s with previous years’ data, we found most links between mid-2018 and mid-2017 (7,000 links) and progressively fewer links to each earlier year’s data (2000 linked to mid-2016 and 1,000 to mid-2015).
To check how representative the findings were for other years, the first stage of the analysis (linking to T minus 1) was repeated using mid-2017 as the base year (see Table 2). The linkage of mid-2017 Flag 4s (used to distribute other migrants) to mid-2016 gave confidence that the patterns observed in mid-2018 were a good representation of the scale of this issue for other years.
|Total other migrant records||180,000||180,000|
|Linked to T-1||7,000||7,000|
|Linked to T-2||2,000||N/A|
|Linked to T-3||1,000||N/A|
Download this table Table 2: Linkage of other migrant records to previous years’ workers and students.xls .csv
Migrant Worker Scan (MWS)
The Migrant Worker Scan (MWS) contains information on all adult overseas nationals who have registered for and been allocated a National Insurance number (NINo). A NINo is generally required by any overseas national looking to work or claim benefits or tax credits in the UK.
Higher Education Statistics Agency (HESA) data
Higher Education Statistics Agency (HESA) data provide the only comprehensive nationally consistent source of data on higher education students. HESA data provide basic demographic data (age and sex); term-time and domicile (parental home) postcode; information on the course (course length and type); and institutional information.
NHS Patient Register
The NHS Patient Register is a record of all persons registered with a GP in England and Wales. The NHS Patient Register was used to maintain an accurate list of all persons registered with a GP, allowing the timely transfer of medical records and correct payments to doctors. It contains a list of everyone who is, or ever has been, registered with a GP in England and Wales since the NHS was founded in July 1948.
The NHS Patient Register holds a “flag” for persons whose previous place of residence was outside of the UK. These data are used (together with other sources) to help allocate separate estimates of international migration into the UK to local authority areas.
Data linkage is a process that temporarily brings together two or more sets of administrative or survey data from different organisations. Match keys are created by putting together pieces of information to create unique keys that can be hashed and used for automated data linkage, with the intention of eliminating some of the discrepancies that might otherwise prevent an automated match. For example, a match key might be constructed from the first three characters of an individual’s forename and surname, combined with their date of birth, sex and postcode district.
The time difference between an event happening (such as an individual migrating between countries) and that event being reflected in administrative data sources is referred to as the lag.Back to table of contents
As shown in Section 7: Impact on local authority distribution of migration, the impact of removing lagged records on the local authority distribution of migrants is small. The most obvious inference from this is that migrants behave in similar ways irrespective of where in England and Wales they migrate to. However, it is worth considering that the impact could be larger if the administrative data more precisely met our target definitions and concepts. One issue is that the administrative data include records of several groups that fall outside of our criteria of long-term international migrants. It is likely that this explains why the 40,000 other non-UK migrants are distributed to local authorities in England and Wales using 180,000 Flag 4s. Some of the reasons for the discrepancy between Flag 4s and the other migrant flow include:
- short-term migrants (those who remain in the country for less than 12 months) applying for GP services
- UK-born returning migrants being issued Flag 4s
- missed links between the Migrant Worker Scan (MWS), Higher Education Statistics Agency (HESA) data and the GP NHS Patient Register
- incorrectly issued Flag 4s (people moving within the UK who are issued with Flag 4s)
The excess records in each data source may be dampening down the impact of removing linked records.Back to table of contents
This work has shown that the method for distributing international immigrants to local authorities in the mid-year estimates has not been negatively impacted by different levels of lagging across administrative data sources.
Further, the immigration distribution as currently used in the production of the mid-year estimates cannot be meaningfully improved at the present time through the inclusion of cross-year linkage of administrative data. While some records are influencing the immigration distribution in multiple years, the removal of records does not result in any significant change in the amount of immigration at the local authority level.
Part of the explanation for the relatively minor impact of making this change is that the other migrant flow is relatively minor compared with the workers and students flows. Given the relatively minor impacts indicated by this research, no changes will be made to the processing of the mid-year estimates.
As part of the Population Statistics and Migration Transformation, further research into better ways of producing immigration and emigration flows are ongoing, and research will continue to be published.Back to table of contents
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