Illegal migrants: can't even get themselves arrested?

How many illegal immigrants are there in the UK? Unlike other such questions - how many 12 year olds are there in the UK? how many gay Jews? - where, although we don't know the exact answer, either survey and administrative data allows us to make an informed and reasonably accurate guess, we don't know, even approximately. But a new initiative by the Metropolitan Police suggests that the number may in fact be surprisingly low. 

The problem with measuring illegal or irregular immigration is that neither surveys nor administrative data get us very far.  People are unlikely to tell even anonymous non-government surveys if their immigration status is questionable. And they don't show up explicitly in any official data set (although they will be counted in some).  So, while most data on immigrants comes from micro data,  attempts to estimate irregular immigration tend to be based on population aggregates, using the residual method: that is, calculating the size of the foreign born population in the UK and working out how many are here legally.  Both numbers can in principle be derived from official data. The difference is the estimate of the irregular population.  

There are two major problems with this, however.  First of all, the estimate of the total foreign born population - from the census, Labour Force Survey, or similar - may well miss some irregular migrants. Second, even if both estimates are reasonably accurate, since the vast majority of the foreign born population are entirely legal, a relatively small proportional error in either (in either direction) leads to a large error in the estimate.

Nevertheless, it's the best we've got, and the last serious estimate in the UK used  this method. Research conducted by Ian Gordon (an acknowledged expert on this topic) and colleagues at LSE for the Greater London Authority in 2009 produced a central estimate of about 620,000, with a range of 420,000 to 860,000.  These numbers seem reasonable enough: certainly it is difficult to imagine that the true number is vastly larger, simply because our population estimates would be wildly and visibly wrong, and it would show up in other official statistics. But they are clearly very uncertain; it would obviously be good to have a much more accurate estimate. 

So what would be the best way to estimate the irregular population?  Well, from a research perspective, the best way would be to take a large random sample of the resident population, UK or foreign national, legal or otherwise, detain them (forcibly if necessary) and oblige them to verify their immigration status. That would do it.  But, fortunately or unfortunately, depending on your perspective, this is not feasible. 

However, with a non-random sample - in particular, people who've been arrested - it is both legal and feasible to do just that. And the Met have. In Operation Nexus, described here, immigration officers have been stationed in every custody suite in London to check the immigration status of everyone arrested:

"When someone is brought into a custody suite for a crime the teams will run identity checks to find out everything they can about the person they have arrested to make sure we take the quickest and most affective action. ... Now the Nexus teams can run checks to establish if foreign nationals who are arrested are wanted abroad, have previous convictions abroad or are here illegally."

So what are the results? Well, first, it is important to sense-check the data by looking at the proportion who are identified as foreign nationals in the first place:  

"In the five week period since the Nexus teams were rolled out in custody suites across London the MPS arrested 25,968 people. 6,988 (27%) were identified as foreign nationals."

Labour Force Survey data suggests that about 27% of Inner London residents, and about 19% of Outer London residents are foreign nationals; the proportion for those who are of criminal age (ie neither too old or too young to be likely to be arrested) will be somewhat higher.  So these data are consistent with foreign nationals being approximately as likely as UK nationals to be arrested in London, which is in turn reasonably consistent with other data (eg prisoner nationality data). If you were estimating the foreign national population in London on the basis of arrest data, you'd not be too far wrong.  Can we do the same with the population of irregular migrants?

Well, we have some numbers.  The Met say:

"155 of these were immediately detained by the UKBA for immigration matters, 25% of which have already been removed from the UK."

In other words, about 0.6% of those arrested (about 2.5% of the foreign nationals arrested) were found to be illegal migrants.  That is a remarkably small number.  0.6% of London's population is about 50,000.  The LSE research estimated that  London accounted for 70% of the irregular migrant population in the UK.  So, at face value, this would translate to a UK-wide estimate of about 70,000 - only a bit more than one tenth of the LSE estimate, and only just over 0.1% of the total population.. 

Now, there are a number of reasons why it might not be appropriate to simply use this estimate - that is, why we might not have a truly random sample:

• irregular migrants might be more or less likely to commit arrestable offences than natives or legally resident foreign nationals.  It's easy to see why they might be more likely; they are already on the wrong side of the law in some sense, and their irregular status may make access to employment or welfare more difficult.  On the other hand, they have a stronger motive than most for staying out of trouble and the hands of the police.  It's not obvious which way the bias goes.  But it seems rather difficult to believe that irregular migrants could really be ten times as law abiding as either the generality of Brits or other, but legal, foreign nationals;

• it could be that there are far more irregular migrants among those arrested than are detected by UKBA officials.  Without knowing exactly what the administrative procedures in custody suites are, it's very difficult to tell. It does seem odd though that having arrested someone, having identified them as a foreign national (and the data there, as we've seen, seem entirely plausible), and having got a UKBA official sitting with them in a custody suite with both the authority and explicit remit to investigate their immigration status, that nine out of ten illegals would go undetected.  

• perhaps, with some migrants, UKBA had doubts (or more) about their migration status but didn't "immediately detain" them, instead (for example) asking them to report back at a later date to demonstrate they were here legally.  

It does seem plausible that UKBA won't have taken immediate action with everyone whose status is irregular; this is common enough with those whose immigration status is found to be questionable in other contexts. But with with those who have actually been arrested it seems considerably less likely (and justifiable), and it's certainly not the impression given by the Minister for Immigration, Mark Harper, who said:  

"I've been clear that we will take all possible action against individuals who pose a risk to the public and remove them from the country at the earliest opportunity. Through our combined work with the police we will use the full force of immigration powers on those who seek to commit crime and damage our communities."

So none of these explanations seem likely to be able to fully explain the discrepancy. It is also useful to compare the data with that from an earlier DWP/Home Office investigation into non-EEA national claiming benefits. This found that  "Of those [records] that were fully matched, 98 per cent were matched to an immigration or nationality status entitling them to benefit."  That is, only 2 per cent were found to be claming benefit despite not being entitled because their immigration status was irregular. (Even that could be an overestimate; you can be here legally  but not be entitled to benefits). 

Again, this is an extremely low number; since less than 5% of the working age population is a non-EEA national, this would correspond to somewhat less than 0.1% of the total population.  At the time, I assumed this mostly reflected the fact that irregular migrants would generally try to avoid the benefit system (and this investigation was not comprehensive - records couldn't be matched for everybody).   However, the numbers are again very low. 

What can we conclude?  I certainly wouldn't argue that 70.000 is a "better" estimate of the UK irregular migrant population than the previously accepted, much larger, numbers; it just seems too low. This is very sketchy, preliminary, and difficult to interpret data.  But it is striking that when we actually have real, hard data, not guesses, about the topics where you would think we should be most worried about irregular or illegal migration - crime, or fraudulent benefit claims  - the numbers of people whose immigration status is found to be irregular is so low.  It must be at least plausible that the level of irregular migration in the UK is considerably lower than we thought.