With migration continuing to make headlines in the media, we unpack what actually defines a "migrant", and how the UK's largest producer of official statistics goes about counting them. We also shed light on the misuse of migration figures.

Transcript

MILES FLETCHER

Hello and a very warm welcome to a new series of Statistically Speaking - the official podcast of the UK Office for National Statistics. This is where we hear from the people producing the nation's most important numbers, about how they do it and what the statistics are saying.

Now it's hard to think of one statistic that could be said to have been more influential these past few years than net international migration. Suffice to say it's the one ONS statistic that probably draws more media attention than any other.

But to fully understand the migration figures, and the swirling debate around them, we'd say it pays to know a little about how they are put together. And the first thing you need to know about that is what, or who, is a migrant in the first place. As usual, to unpack and explain the migration statistics we have the top experts from the ONS and beyond. Mary Gregory is director of population statistics here at the ONS. Madeleine Sumption is director of the Migration Observatory at the University of Oxford, and new chair of the National Statistician's Advisory Panel on Migration. And to help us understand how the numbers are used and abused in public debate, we're also delighted to welcome Hannah Smith, senior political journalist at fact checking charity Full Fact. Welcome to you all.

Madeleine, to start with you if I may, with that fundamental question, quite simply, what is a migrant? MADELEINE SUMPTION

Well, there are actually lots of different definitions of a migrant and we use different definitions at different points in time. The standard definition of migration that we use in this country is long term migration, so people moving for at least 12 months, and so the Office for National Statistics figures on immigration, emigration and net migration are all using that definition. And people in that data, they're migrants regardless of whether they are British or not British. So you could have a British person who's gone to live overseas for a few years and coming back they would be counted, in theory, at least in the data, as a migrant. There are other definitions though that are very useful for policy. So sometimes people talk about migrants, meaning people who don't have British citizenship, and the value of that is that these are people who are subject to immigration control, effectively that the Home Office is regulating their status. But it's also sometimes quite useful not to look at whether someone's a citizen now, because of course people can change their citizenship, and many migrants to the UK do become British citizens. So it can be useful to look at whether someone has migrated in the past. The standard definition for that is whether someone is born abroad. But now we've got all these exciting new data sets from administrative data, and so there's a new definition that's creeping in and being used a lot, which is someone who was a non-citizen at the time they registered for their National Insurance Number, regardless of whether they've subsequently become a British citizen. So it's a bit confusing sometimes for the external user, because for various reasons, we have to have all of these different definitions. You just have to know which one you're looking at at any point in time. MILES FLETCHER

But the basic headline definition, as far as the ONS is concerned -and I guess internationally too because it's important that these figures are comparable- is that it is a person traveling from one country to another for a period of 12 months.

MADELEINE SUMPTION

That's right

MILES FLETCHER

And I guess that is something that is perhaps not widely understood. People understand that migration has a degree of permanence, so they move from one country to another, and yet you can be a migrant in quite a sort of transient way. MADELEINE SUMPTION

That's right, we have short term migrants as well. So we have a lot of people who come to this country to do seasonal work. For example, they spend up to six months in the country. Then you have people who are long term migrants by the ONS definition and they may spend two to three years here, for example, if they're a worker or an international student. So you're right. I think in people's minds, often when they think about who is a migrant and who comes to mind, they will typically think of someone who is moving permanently. But actually a lot of migrants to the UK only stay for a couple of years. MILES FLETCHER

And none of these people, when it comes to measuring them, none of these people arrive Paddington Bear style with labels around their necks saying "I am a migrant". The ONS in measuring migration has to classify whether these people qualify or not. MADELEINE SUMPTION

That's true, and that is very tricky. And it's something I think the non-technical user of the statistics finds it difficult to appreciate quite how hard it is for ONS to work out who is a migrant or not. Because we have millions of people crossing our borders every year, most of them not migrants. We have tourists or people who come in to visit family members. There are all sorts of people and reasons why those people come and go, so ONS is really looking for the needle in the haystack, and a relatively small share of people who are crossing the borders are actually migrating. MILES FLETCHER

Well, that seems a good moment to bring in the person who is in charge of finding that needle in the haystack statistically. Mary, tell us how we approach this task. Perhaps start off by explaining how we used to do it.

MARY GREGORY

Previously we used a survey called the International Passenger Survey, and there we would ask a sample of people as they came into the country, or as they left the country, what their intentions were, and we'd be able to provide very early estimates based on people's intentions to stay or to leave. MILES FLETCHER

This is people at airports and other ports of entry, ferry ports, for example, simply approaching people as they wander along the corridors, almost in a random sort of way. MARY GREGORY

Exactly that, you might have seen them. If you have travelled through an airport you may have seen a desk that sometimes says Office for National Statistics. And there would sometimes be people there with very carefully scheduled timetables to make sure that we collect a good cross section of people. MILES FLETCHER

So the International Passenger Survey is essentially a big sample survey. Nothing wrong with that, and yet, the number of people being stopped at airports who did actually identify themselves as being migrants was quite small, and that made for some very broad-brush estimates didn't it? MARY GREGORY

Yes, as you can imagine, people travel through airports or ports for many different reasons, and a lot of those people traveling will be traveling for a holiday or business or to visit family. And so the proportion of those people who are actually going to become residents or leaving for more than 12 months is very small, which makes it really difficult to pick up a good sample of those people. MILES FLETCHER

And because it's fair to say the International Passenger Survey was never set up to measure migration in the first place, and that was something ONS found itself pointing out for a long, long time before things actually changed.

MARY GREGORY

For a number of years we made clear that it was being stretched beyond its original purpose, and that it was the best measure we had at the time but now we think we can do better. And I think one other really important aspect of that is understanding that the survey was asking about people's intentions, and intentions don't always match reality of what we then see. MILES FLETCHER

Because you might arrive as a student, end up working, settling, starting a family... MARY GREGORY

Yes. Or you might find that you've arrived planning to stay for a year and then change your mind and you've left again. So it could go in either direction. MILES FLETCHER

So the case for change was strong. What has changed? How is migration measured now? MARY GREGORY

So now we have a variety of different ways to measure depending on the nationality of the people ...

Migration: The needle in the data haystack