Full Fact: Why official estimates about the number of student migrants staying in the UK were wrong

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How many non-EU students are staying in the UK illegally?

This week the Home Office published data on exit checks — gathering information on people as they leave the UK.

That data has told us a lot we didn’t previously know about how long people stay in the UK before leaving.

Why the fuss about non-EU students?

Non-EU student immigration has been a focus of immigration policy for several years. Since 2010 the government has introduced stricter rules for admitting non-EU students to the UK. Theresa May oversaw this as Home Secretary and has since called for further restrictions as Prime Minister.

Non-EU nationals are thought to make up about 70% of student immigration to the UK and they’re subject to immigration controls. Students from EU countries don’t currently need a visa to come to the UK so we have less data about what happens to them.

Even so, before today we didn’t actually know how many international students returned home after studying in the UK. The available evidence was contradictory.

On the one hand, ONS figures were telling us that relatively few former student immigrants actually seemed to be emigrating abroad after finishing their study. On the other, Home Office visa data indicated that most were likely to be leaving.

This week's exit checks figures firmly took the Home Office’s side. The vast majority of students here on a visa seem to be returning home.

That, in turn, has raised new questions about how useful the ONS estimates are and how they can be improved in future.

We’ve been underestimating how many international students leave the UK

In the last year, most international students have been leaving the UK once their visas have expired. In other words, non-EU students usually comply with the terms of their visas.

69% of students who previously came to the UK on a long-term visa left once that visa expired in 2016/17. Another 26% extended their visas to remain in the UK and the rest either fell off the radar or appeared to leave after their visa had expired.

Not all of those people will have left permanently. Data from the previous year – 2015/16 – shows that 21% of those departing the UK returned next year on a short or long-term visa. Most, however, had not returned to the UK.

Why have we been getting it wrong?

Put simply, stated intentions aren’t the same as outcomes.

ONS student migration figures are based on asking people arriving and leaving the UK why they came here.

There are two main reasons why the statisticians think we might have been getting this wrong. Firstly, when leaving the UK people don’t always accurately recount when and why they first came to the UK. They may stay on after their studies for work, and not report study as their initial reason, for example.

Secondly, people don’t know what they’re going to do next and so might not end up doing what they say they’re planning in a survey.

So if we’ve been overestimating how many students stay in the UK, does this mean that overall migration estimates are wrong?

Given what we’ve said above, it may seem likely, but the ONS says not necessarily.

That’s because if we’re getting students’ intentions wrong, we could be getting other people’s intentions wrong too, and we don’t know how all those errors might add up.

The ONS says this means “further investigation” is needed before any conclusions can be reached. It plans to look again at how it adjusts overall migration figures to account for the fact that people may change their intentions as stated in the survey. It says it will also continue ongoing work to improve the estimates and use new data sources to better inform the wider debate.

In any case, it adds doubt to how reliable the overall estimates are.

For now, it warns that its own intentions figures aren’t robust enough and that the Home Office exit checks data provides a more accurate picture of what non-EU students do after their visas expire.

So shouldn’t we just use exit checks data from now on?

Counting people as they leave the UK can’t solve everything. For one thing, you don’t know how long someone intends to stay abroad.

If they don’t come back within a year, they’re defined as an emigrant, whereas, if they come back before a year has passed they were only a ‘short’ term migrant or just visiting abroad. A survey can give you a window on that – pure exit data can’t do that straight away.

None of this tells us anything about EU or UK national students either so we can’t yet get a complete picture of how many students are entering and leaving the UK.