1. How can we measure illegal migration?

There are currently no up-to-date estimates for the number of illegal migrants’ resident in the UK. By its very nature, it is extremely difficult to know the exact size of the illegally resident population and due to the challenges in making reliable estimates the government has not produced any official estimates since the mid-2000s. Nevertheless, the Office for National Statistics (ONS) and Home Office statisticians have been working with departments across the Government Statistical Service and academics to explore possible approaches to the measurement of illegal migration in the UK. Here Jay Lindop and Jon Simmons provide an update on the progress made so far.

In February 2019, the ONS Centre for International Migration and the Home Office held a workshop which included leading academics and experts from government to discuss the measurement of illegal migration. The workshop explored the different definitions of illegal migrants, whether illegal migration could be measured using existing data sources and the possibility of using admin data to measure illegal migration in the future.

The definition and coverage of illegal migration is complex, there are many individual populations that can be included, such as:

  • illegal entrants
  • overstayers
  • those with failed asylum claims
  • those not adhering to the conditions of their visas

It was also noted that whilst some people remain in the UK without legal authority other individuals may find themselves in an irregular and unresolved status, which may prove to be temporary.

There was some discussion in the workshop around the best way to present future estimates of illegal migration in the context of the range of possible definitions.

The last official estimate of illegal migration was published by the Home Office in 2005 and measured the “illegally resident population”. This definition included those who entered the UK illegally (illegal entrants) and those who entered the UK legally but subsequently fell into illegal status when their visa expired (overstayers). This is a widely accepted definition of illegal migration, however with increasing access to admin data there may be an opportunity to widen this definition in the future.

The discussion considered two main approaches for the estimation of illegal migration. Firstly, to try to create one overall estimate like the one published by the Home Office in 2005. Due to new methodologies and access to administrative data this estimate would not be comparable to the one previously produced and there were a variety of issues which would affect the usefulness of any one number estimate. The second and preferred option is to identify individual components of illegal migration and attempt to understand each using the most appropriate data sources, which might not be comparable nor would necessarily be able to provide estimates of numbers.

Office for National Statistics has previously published an update on ONS’ progress in putting administrative data at the core of migration statistics. The increased access to administrative data sources that this programme entails will potentially also help government measure previously unidentifiable populations such as illegal migrants. The programme is still in its early stages and so the ONS does not currently have access to all the administrative data that would be needed to fully test a method of identifying illegal migrants, however it was proposed the ONS does some exploratory analysis using the existing data.

One potential method would be to run a pilot study using a sample of illegal migrants already known to the Home Office. By carrying out this research we can use a sample of illegal immigrants and, using linked datasets, see how they interacted with the administrative data and thereby in that way investigate what might be known about their existence in the UK. This might also allow us to explore whether we could develop methods to identify this population in the future.

Any proposed research of this nature would need to be considered by the National Statistician’s Data Ethics Advisory Committee (NSDEC). NSDEC was established to advise the National Statistician that the access, use and sharing of public data, for research and statistical purposes, is ethical and for the public good.

If this exploratory research is approved, we will work with other departments and academics to progress it. Irrespective of the outcome of the Ethics Committee decision, we will also be considering other research on illegal migration. For example: exploring the use of temporary National Insurance numbers by illegal migrants; and investigating whether there are any new methods being used around the world to measure illegal migration, building on the investigations reported on in the 2005 Home Office reports.

We will provide a further update on where we have got to with this work once it is underway.

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Contact details for this Article

Jay Lindop
Telephone: +44 (0) 1329 444097