DOGE, apples and irregular migrants

Summary

More or Less: Behind the Stats

Episode: DOGE, apples and irregular migrants
Host: Tim Harford, BBC Radio 4
Original Air Date: March 12, 2025

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Episode Overview

In this wide-ranging episode, Tim Harford and the More or Less team dissect several topical numerical claims:

• The credibility of cost-saving claims from the US Department of Government Efficiency (DOGE), led by a fictional President Trump and advisor Elon Musk
• Whether VAT on private school fees meaningfully affects UK inflation figures
• Are New Zealand apples really greener than UK-grown ones?
• Does one in 12 Londoners truly count as an "illegal" or irregular migrant?

Each topic is examined with More or Less’s trademark wit, skepticism, and rigorous fact-checking, featuring interviews with experts and careful breakdown of the relevant statistics.

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Key Discussion Points and Insights

1. DOGE: Department of Government Efficiency Cost-Saving Claims

(01:45–10:27)

• Background:
  President Trump claims DOGE, led by special advisor Elon Musk, could save the US $2 trillion a year.

  • Linda Bilmez, budget-balancing economist under Clinton, called that “implausible”:

    “The only way you could cut $2 trillion out of the budget in a year is... in the shooting yourself in the head kind of mechanism.” (03:01)

• Fact-checked “savings” included:

  • Alleged $50 million to Gaza for condoms for Hamas. Tim Harford reveals that:

    “There is no money, as far as I can tell, going to Gaza for condom procurement.” (04:20)

  • Error stemmed from confusion between Gaza field hospitals and Gaza province in Mozambique.

• Social Security “fraud” claims:

  • Musk suggested Social Security might be the “biggest source of fraud in the whole of human history.” (05:01)
  • The cited figure—that there are 18.9 million Americans over 100—is a data quirk, not reality; defaults in birth dates, not fraud.
  • This does open the door to some fraud, especially immigrants using dead people’s numbers to pay taxes; but “billions have not been saved.” (06:17)

• The “transgender mice” grant controversy:

  • $8 million supposedly spent on “making mice transgender” is mostly misrepresented research on hormones and unrelated “transgenic” (genetically engineered) mice, not “transgender.”

    “The mice are also not transmorphic, translucent or transformers.” (07:20)

• DOGE’s Wall of Receipts:

  • Many contracts were misrepresented as savings, including overstating an $8 million contract as $8 billion. More than 1,000 contracts altered or erased on their public list.
  • Claiming $15 billion in savings (less than 1% of target), but much of this is questionable; many staff are on “paid administrative leave”—no savings, no productivity.

• Elon Musk’s admission:

  “Some of the things that I say will be incorrect and should be corrected.” (10:23)

Memorable moment:
Tim's sardonic take on government “efficiency” statistics, the absurdity of the “condoms for Hamas” example, and deadpan fact-checking throughout.

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2. VAT on Private School Fees and Inflation

(10:27–14:26)

• Public concern: VAT on private school fees reportedly increased UK inflation—how much did it really contribute?

• Stephen Burgess from the Office for National Statistics explains:

  “Inflation... rose from 3.5% in December to 3.9% in January... of that we think 0.06 was due to private school fees, with bigger contributions coming from food and airfares.” (11:36)

• Private school fees are included in inflation metrics because they account for significant total spending, despite only 6% of children attending independent schools.

  • Example: “We have a threshold of about £400m of expenditure... private school fees are well above that.” (12:38)
  • The so-called “basket” of goods includes items based on aggregate spending, not household prevalence.

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3. Are New Zealand Apples More Environmentally Friendly than UK Apples?

(16:05–22:16)

• Claim: “Carbon emissions from New Zealand grown apples are 32% lower than apples grown domestically, including emissions from shipping.” (16:17)

• Based on a 2006 New Zealand study—a time when “food miles” were a hot topic.

• Imported apples from NZ: 185kg CO2/tonne vs. UK apples: 272kg CO2/tonne—mainly thanks to higher orchard yields and sunshine.

• Dr. Hannah Ritchie (Our World in Data, Oxford):

  “Transport only contributes around 5 to 6% globally [to food-related emissions]... for apples, it's around 20%... apples are a relatively low carbon food.” (17:57, 18:43)

• Caveats: The comparison depends on season and storage:

  • The 2006 study assumes apples are consumed immediately, but most are refrigerated. The UK’s electricity now is much greener, closing the emissions gap.
  • Dr. Ritchie:

    “Emissions per unit of electricity in the UK have more than halved... because we basically got rid of coal.” (21:39)

• Conclusion:

  • The claim was “questionable back in 2006 and is definitely not true today.” (22:16)
  • Best practice: Eat British apples in winter, NZ apples in spring.

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4. Is 1 in 12 Londoners an Irregular Migrant?

(23:18–29:54)

• Headline: Daily Telegraph claimed “up to one in 12 in London is an illegal migrant."

  • Number came from a Thames Water internal report—not from the ONS.

• Charlotte MacDonald breaks down the faults:

  • Thames Water's consultants used a 2017 Pew Research Center estimate, applying the “residual method” (subtracting known legal residents from total foreign-born population).
  • Problems:
    • Calculation errors: Telegraph had to revise to 1 in 13; midrange is 1 in 15.
    • Pew likely undercounted people with “leave to remain.”

      “Since 2004, over 860,000 people have been granted leave to remain... at least 280,000 haven’t claimed citizenship.” (26:39)

    • Inadequate subtraction leads to overcounting.
    • “Educated guesses built on educated guesses”; data is now eight years old; much of the ‘irregular’ population is miscategorized.

• Other data points:

  • Police administrative data: “Proportion of irregular migrants was absolutely tiny, down at the sort of 1% level." (Jonathan Portes, 29:07)
  • Increases in legal migration since 2017 render the old estimate less reliable.

• Expert quote:

  • Meekna Kwibis, Migration Observatory, University of Oxford:

    “The simple answer is that we simply don’t know how many unauthorized migrants there are... there is no data ... to say with any degree of precision what the actual number is.” (27:53)

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Notable Quotes & Memorable Moments

• On budget cuts:

  “The only way you could cut $2 trillion out of the budget in a year is... in the shooting yourself in the head kind of mechanism.”
  — Linda Bilmez (03:01)

• On fraud and dead Americans’ social security payments:

  “If the date of birth is not known, the computer system defaults to a date of birth of 20th May, 1875, under the star sign, appropriately enough, of Taurus the Bull.”
  — Tim Harford (05:28)

• Government press release tone:

  “What interesting language for an official governmental statement.”
  — Tim Harford, after reading Trump’s mouse research statement (07:20)

• On statistical methods:

  “Educated guesses built on educated guesses”
  — Tim Harford, on ‘irregular migrant’ estimates (28:19)

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Timestamps for Important Segments

• DOGE cost-saving claims: 01:45–10:27
• VAT on private schools & inflation: 10:27–14:26
• New Zealand apples' carbon footprint: 16:05–22:16
• London “illegal” migrants claim: 23:18–29:54

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Episode Takeaways

• Major government and media statistical claims are often exaggerated, miscalculated, or based on outdated/inadequate data.
• Even credible sources can have systemic flaws—such as not subtracting all those legally present from irregular migrant estimates.
• Old environmental statistics (like “food miles” from 2006) are less meaningful now, especially with energy grid decarbonization.
• Inflation metrics reflect total expenditure, not just widespread usage—hence private school fees matter more than you’d guess from household usage alone.

The episode’s tone is dry, lightly sardonic, but rigorously factual. Perfect for listeners who want to get past headlines to the real numbers—and some polite statistical debunking.

Transcript

[Tim Harford]

This BBC podcast is supported by ads outside the uk.

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[Dr. Hannah Ritchie]

BBC.

[Charlotte MacDonald]

Sounds music radio Podcasts hello and welcome.

[Tim Harford]

To more or less. We are the very model of a modern new statistics show expect economics, fact checks and many cheerful facts about the square of the hypotenuse. And in particular expect us to investigate the claim that one in 12 Londoners is is an illegal immigrant, which comes from. Well, not the source you might expect. And the claim that New Zealand apples are particularly environmentally friendly, which comes from exactly the source you might expect, namely New Zealand. Our loyal listeners are wondering whether VAT on private schools really has boosted the rate of inflation or if that's just a media concoction. But first, it's been 12 weeks since President Trump announced the formation of DOGE, the so called Department of Government Efficiency. Led or not led, but definitely quite led by his special advisor Elon Musk. Musk claimed that he could save the US government $2 trillion a year. We asked Linda Bilmez, someone who was part of the team who actually balanced the US budget to under President Clinton, whether this was plausible.

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The 2 trillion number over a decade is a realistic goal, but the only.

[Charlotte MacDonald]

Way you could cut 2 trillion out of the budget in a year is.

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In the shooting yourself in the head kind of mechanism.

[Tim Harford]

Cool. Since then, there have been a raft of cuts to public spending and claims about various acts of fraud that probably haven't happened. To celebrate their achievement, we have compiled a lovely little presentation I do hope you enjoy. In the last week of January, the USAID website went offline and thousands of US workers abroad were told to return home. President Trump announced that the Department of Government Efficiency had saved the US tons of money, including we identified and stopped $50 million being sent to Gaza to buy condoms for Hamas. 50 million. There is a report, or was a report, of all of their condom procurement that showed that none of it had gone to Gaza. That report has now been taken down along with the entire US Agency for International Development website.

[Stephen Burgess]

So they pointed at one point to funding that was going to field hospitals in Gaza. That was about $50 million, but of course that was for field hospitals. I have no idea where they got the notion that it was for condoms.

[Tim Harford]

There is no money, as far as I can tell, going to Gaza for condom procurement. There are a number of different Gaza related options that, you know, this could actually be bull. Funding that is going to Gaza and funding that is going to the Gaza province of Mozambique. The whole condom statistical mishap was inconceivable, but not unique. On to the next one.

[Stephen Burgess]

There's crazy things like just cursory examination of Social Security and we've got people in there that are 150 years old now. Do you know anyone who's 150? I don't. Okay.

[Tim Harford]

Elon Musk. He later tweeted that the Social Security Administration in the United States might be the biggest source of fraud in the whole of human history.

[Charlotte MacDonald]

Perhaps the most shocking was the revelation that deceased people who would have been 120 to 150 years old are still receiving Social Security payments totaling billions of dollars per month.

[Tim Harford]

This turned out to be just a programming quirk. If the date of birth is not known, the computer system defaults to a date of birth of 20th May, 1875, under the star sign, appropriately enough, of Taurus the Bull. More generally, there is a problem. Some people just don't have their deaths recorded in the database. Which means that according to the Department of Social Security, There are currently 18.9 million people aged over 100 in the United States. Which is definitely not right. And this opens the door to potential fraud. But two reports from the Inspector General found.

[Charlotte MacDonald]

Officials also noted that almost none of.

[Dr. Hannah Ritchie]

The 18.9 million number holders currently receive SSA payments instead.

[Tim Harford]

The type of fraud that occurs is largely immigrants using the Social Security numbers of dead people to pay taxes so they can get jobs. So again, billions have not been saved. On to the next has the government.

[Stephen Burgess]

Really spent $8 million on making mice transgender?

[Tim Harford]

$8 million for making mice transgender?

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I don't think it means what you think it means.

[Tim Harford]

This is real. When challenged on this claim, the White House produced a list of studies whose grants all added up to around $8 million.

[Stephen Burgess]

Last night, President Donald J. Trump highlighted many of the egregious examples of waste, fraud and abuse funded by American tax, including $8 million spent by the Biden administration for making mice transgender. The fake news losers at CNN immediately tried to fact check it, but President Trump was right as usual.

[Tim Harford]

What interesting language for an official governmental statement. Anyway, we've been through the list and this is what we've found. Most of the studies were looking at the impacts of hormone therapy. One such study looked at whether people born male who took estrogen supplements and would be more at risk of getting breast cancer. Another looked into whether these medicines could change the effectiveness of a potential HIV vaccine. One of the trials studied the effects of hormones more generally looking at the link between oestrogen and asthma. And one of the studies, which cost more than a million of this supposed $8 million, was nothing to do with hormones at all. It involved an entirely different form of trans mice, transgenic mice, meaning genetically engineered, not transgender. The mice are also not transmorphic, translucent or transformers. As part of their effort to prove to the US how incredibly efficient they're being, Doge posted a list called the Wall of Receipts detailing contracts they have cancelled and and what savings those represented. But soon after, the five biggest savings on that original list were deleted after members of the press pointed out errors. One of these so called savings was cutting a contract Doge said was worth $8 billion. Although it turns out the contract was actually worth $8 million, which to the nearest percent is 0% of 8 billion. By early March, Doge had erased or altered more than 1,000 contracts on this WAL receipts more than 40% of all the contracts posted. This includes our next number. Doge claimed that they'd cancelled a $133 million US aid contract to an agency in Libya. However, this contract had actually come to its end under the Biden administration. So again, no $1.9 billion. Wowzer. Doge claimed that they'd severed an Internal Revenue Service contract for a company that provides assistance with Technology only Again, this contract had been cancelled during the Biden administration. So how much has Elon actually saved the US government? In the category of contracts, Doge are now claiming $15 billion in savings, which is less than 1% of his $2 trillion target for government agencies. It's difficult to say as most staff whose jobs are potentially being cut are being put on paid administrative leave. So they're paying people, but they're not getting any productivity or output out of them. Now, Elon Musk has had some of these mistakes pointed out to him and he's acknowledged them, such as the $50 million worth of condoms.

[Stephen Burgess]

Well, first of all, some of the things that I say will be incorrect and should be corrected.

[Tim Harford]

Agreed. So the fact checking losers at More or Less will keep checking those facts and we'll keep you abreast of what we find. If you want updates all year round, even when we're not on Radio 4, you will find them on our weekend podcast. You're listening to more or less. UK inflation has gone up from 2.5% to 3%. And it was reported that this increase was driven by a rise in airfares, food and adding VAT to private school fees. But a number of our listeners emailed in to check if that last one was right. They asked, can the new VAT requirements have really made a difference when private schools are only used by a small proportion of the population? Or are the newspapers just mentioning it because it's topical? Or possibly because some newspaper editors send their kids to private school? To find out the truth, we spoke to Stephen Burgess from the Office for National Statistics. They're the people responsible for calculating inflation. I started by asking him, how much of an impact did VAT on private schools really have on inflation?

[Stephen Burgess]

Inflation on the CPIH measure, which is our broadest measure, rose from 3.5% in December to 3.9% in January. So that's a rise of about 0.4. And of that we think 0.06 was due to private school fees, with bigger contributions coming from food and from airfares.

[Tim Harford]

So private schools were responsible for almost a sixth of the total change then? Enough to justify being mentioned in a news article. But before we dig into this, I suppose we should first establish what did the ONS do to calculate inflation.

[Stephen Burgess]

What we're trying to do is to track the average price of all the goods and services that all consumers of the UK buy and how that's changing over time. In order to do that, we'd also get information about prices we collect Thousands and thousands of prices from a variety of shops and services as well. And you can imagine a sort of basket of goods and services that is fixed over time, and we track the prices in that.

[Tim Harford]

So why are private schools included in this basket if not many people use them?

[Stephen Burgess]

I think the important point here is it's about total household spending across all consumers, not what you might think of as the kind of dialysis representative household. So actually, smoking is quite a good example on this. We think only about 12% of adults in the UK smoke nowadays, but still we spend about 20 billion pounds collectively in the UK on cigarettes and other tobacco products each year, which is about one and a half percent of all our spending. So that's significant enough for it to go in the inflation basket. And it's very similar for private school fees. As you say, we think only about 6% of children attend independent schools. But for those families that do send their children there, that's enough spending to justify including in the basket. So, informally, we have a sort of threshold of about 400 million pounds of expenditure that an item has to see in a year before we think about including it. So private school fees are well above that threshold.

[Tim Harford]

So we're not trying to say, okay, what is the average consumer spend or what is the typical consumer spend? We're trying to get the average of all the spending, which is not quite the same thing. And people who do send their children to private schools and they're spending probably tens of thousands of pounds. So if there's a 20% increase or anything close to a 20% increase in that, that's going to show up in the overall statistics, even though the actual number of people involved is quite small. But exactly how is the impact to private school fees worked out?

[Stephen Burgess]

There's about 700 representative items that we track. And then a really important point is that we have to apply weight to those. So not every good or service gets an equal amount of spending. In the uk, for example, we spend about twice as much collectively on food as we do on clothing. So we have to reflect that when we aggregate the figures.

[Tim Harford]

Yeah. So if the price of food went up, that would count twice as much towards the overall basket as if the price of clothing went up because people spend twice as much on food.

[Stephen Burgess]

Yes.

[Tim Harford]

Thank you, Stephen Burgess from the Office for National Statistics.

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[Tim Harford]

Sticks the other day our editor came across an interesting claim on a Twitter thread about the carbon dioxide emissions associated with producing various foodstuffs in the UK.

[Charlotte MacDonald]

Carbon emissions from New Zealand grown apples are 32% lower than apples grown domestically, including emissions from shipping.

[Tim Harford]

Interesting. Now, I've heard similar claims before about New Zealand lamb and about Spanish tomatoes. You may need to ship them from Spain, but you don't need to heat the greenhouses when you grow them. But apples? I always thought apples were kind of our thing. After all, it was allegedly an English apple that discovered gravity in 1666. So are new Zealand apples more green than English ones? Other colours of apple are available red or deliciously golden or pink or ahem. Well, the claim comes from a paper published in 2006 titled Food Miles and the Impact on Carbon Footprinting and Their Potential Impact on Trade. The paper was produced by a team from New Zealand who realised that people were starting to worry about the environmental impact of food miles, which is a problem for New Zealand because they make a lot of food a lot of miles away from everywhere else. This report calculates all of the energy used during the cultivation and production stage and gives it a CO2 equivalent per tonne of apple produced. The report found that even including shipping, apples imported to the UK from New Zealand cost 185kg of CO2 for every tonne of apple versus 272kg for varieties grown in the UK. So 32% less. That seems shocking to some, but not to everyone.

[Dr. Hannah Ritchie]

So if you look at globally emissions from the food system, food miles or food transport only contributes around 5 to 6% globally. So it's actually a much, much smaller fraction than I think many people would assume.

[Tim Harford]

Ah, our friend, Dr. Hannah Ritchie, Deputy editor of Our World in Data and senior researcher at of Oxford. Food can contribute to climate change in lots of ways, from the methane burps of cows and sheep to the chainsawing of forests to allow animals to graze, to the energy required to produce fertiliser. But apple production is relatively low emission, which means that for apples, transport may be a more significant proportion of the environmental footprint as a global average for.

[Dr. Hannah Ritchie]

Apples it's around 20%. And one of the reasons for that is that the emissions that are associated with land use and emissions on the farm for apples are actually very low compared to many other food choices. So if you think about it, even if the amount of emissions associated with transporting a kilogram of beef and a kilogram of apples is the same, right, say we're shipping beef and apples all the way from New Zealand and the emissions associated that is the same, you would expect that as a share of the total carbon footprint of a food transport will be much lower for beef than it is for apples. And that's because the total amount of emissions associated with the land use change and producing the food is much higher for beef than it is for apples. So this 20% transport point for apples is actually just a very clear indication that, that the carbon footprint of apples, whether it's UK based, whether it's New Zealand based, is actually very, very low. Right? Apples are a relatively low carbon food.

[Tim Harford]

It's probably unsurprising that apples are a low carbon food as they grow on trees, the so called lungs of the earth. But if they're so low carbon, why on earth would it be more environmentally beneficial to ship them to somewhere else? The simple answer is that New Zealand has the potential to produce more apples in, in less space. Something to do with the fact that they have more sun feels like cheating. And that means the potential to use less fuel and less fertilizer.

[Dr. Hannah Ritchie]

And there are maybe two main reasons why, or the two main reasons in the study why you might assume producing apples in the UK emits more carbon than New Zealand. And one of the factors is yield and productivity of farming. So we know that apple yields in New Zealand are about 30% higher than they are in the UK. I think when you're comparing yield, one is the total amount of produce that you can produce for a given unit of area of land. I think what's also key for productivity is that if you have very, very productive plants, often you can get a larger number of apples for less inputs. And in this case, less inputs might be fertiliser use. Right. So if you have a better climate, if you maybe have better apple varieties that are more productive, you might need less of, say, fertilizer to produce a given amount of apples.

[Tim Harford]

Six months in a fridge is a lot of electricity, but we're not sure the comparison is quite fair. The report from New Zealand assumed New Zealand apples are consumed as soon as they arrive in the uk. That's not true. New Zealand apples will also often be refrigerated until needed. And research from 2006 is missing something important about the cost of all that refrigeration.

[Dr. Hannah Ritchie]

The UK has really cleaned up its electricity grid, so emissions per unit of electricity in the UK have more than halved. And a big part of that is because we basically got rid of coal. We were still getting around one third of our electricity from coal, and coal is obviously the worst fuel you can really use in terms of producing electricity for its climate impact. And we've now basically got rid of coal completely from the grid. So I think these two factors probably have closed the gap between the emissions associated with New Zealand apples and UK apples.

[Tim Harford]

Dr. Hannah Ritchie, author of not the End of the World. We contacted Caroline Saunders, the lead on the original paper, and ex director of the Agribusiness and Economics Unit at the University of Lincoln. She agreed that the paper was written a long time ago, so things would be different now. We think it can make environmental sense to eat apples from New Zealand in the uk, but we also think the idea that imported New Zealand apples contribute almost a Third Third Less CO2 emissions was questionable back in 2006 and is definitely not true today. In fact, any comparison is potentially misleading because the emissions are so dependent on the season. If you want to eat apples in the most responsible way, eat British apples in the winter and New Zealand apples in the spring. And in the meantime, maybe have a biscuit. Several loyal listeners got in touch about a claim they'd seen in various newspapers. Patient Zero, it turns out, was the front page of the Telegraph.

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Up to one in 12 in London.

[Charlotte MacDonald]

Is an illegal migrant.

[Tim Harford]

That seems like a big number if true. But is it true? Here to tell us more is our very own Charlotte MacDonald. Hello, Charlotte.

[Charlotte MacDonald]

Hello, Tim.

[Tim Harford]

So, Charlotte, where did the Telegraph get this number?

[Charlotte MacDonald]

Well, not from the Office for National Statistics. It was actually based on a study commissioned for internal use only by Thames Water.

[Tim Harford]

Thames Water?

[Charlotte MacDonald]

Yeah, they wanted to figure out how many people live in the area they supply, which, by the way, isn't the same as the official boundary of London. But let's let that one slide. That estimate needed to include people such as tourists, people with second homes, refugees from Ukraine and also unauthorized immigrants. And the Telegraph got their hands on the report that Thames Water commissioned.

[Tim Harford]

And do we believe the numbers?

[Charlotte MacDonald]

Well, we don't believe the maths. The Telegraph made one fairly elementary maths error and had to correct the ratio from 1 in 12 to 1 in 13. Once that was pointed out. And they also rather cheekily looked at a range of estimates in the Thames Water report and picked the biggest. The mid range would put the ratio at 1 in 15.

[Tim Harford]

Hmm. Well, I suppose we should have seen that one coming with the phrase up to. Up to anything always means it could be this big. But it probably isn't. But Charlotte, what I'm really wondering is how reliable Thames Water's underlying numbers are. I mean, Thames Water are famous for. Well, it'd be cruel to say what they're famous for, but let's just say they're a water company, they're not a census bureau. So where did they get their numbers and can we trust them?

[Charlotte MacDonald]

The underlying data comes from a respected American think tank called the Pew Research Center. But Pew's numbers were based on data from back in 2017. They were also for the UK as a whole. So Thames Waters consultants Edge analytics had to make various assumptions to get a number for London.

[Tim Harford]

And where did Pew get their figure from?

[Charlotte MacDonald]

Well, the Pew Research Centre estimated that there were about a million irregular migrants in the UK back in 2017, plus or minus a couple of hundred thousand. They derived that estimate using something called the residual method. That's where you take the official statistic for the number of migrants in a country's population, in this case, the number of non EU nationals. Then using official data, still you remove all the people you know have a legal right to be in the country. So people with visas to study and to work, then those with residency rights in the country. And then once you've removed all the people with the right to be in the country, the number you have left, the residual is the amount they believe is the irregular migrant population. But the key thing with this method is you must make sure you subtract all the relevant groups. If you want to end up with the right answer, and we're not sure they did.

[Tim Harford]

Tell me more.

[Charlotte MacDonald]

Well, one of the biggest criticisms of the Pew research is that they miss out a large group of people who have the right to be in the uk legally. We think Pew got their numbers from the UK submission to the EU statistical office, Eurostat. And because the UK doesn't have a formal population registration system, unlike many EU countries, the UK's numbers weren't easy to interpret. They included people with visas, but not foreign nationals with something called leave to remain.

[Tim Harford]

Well, that sounds like a pretty massive oversight.

[Charlotte MacDonald]

Yeah. And we're not sure that's what Pew did, but several experts think that's what happened, and we've invited Pew to tell us otherwise if they wish, and they haven't so far.

[Tim Harford]

Would it make a big difference?

[Charlotte MacDonald]

Well, yeah, but we don't know how big. The Windrush scandal is a reminder that the Home Office record keeping hasn't always been great. We think that since 2004, over 860,000 people have been granted leave to remain, and we know that at least 280,000 have not gone on to claim citizenship. The ONS estimates that around 250,000 people were given leave to remain before 1993. But some of these people will have died or left the uk.

[Tim Harford]

So, as patchy as it is, you do have some numbers for people with leave to remain, and we're talking about hundreds of thousands of people. So that means they haven't done enough subtraction to come up with the right answer.

[Charlotte MacDonald]

Yeah. But on the other hand, this method is probably undercounting the number of people here because certain groups don't want to be included and are probably trying to avoid being counted, although researchers do try to account for that in their estimates. Here's Meekna Kwibis, a researcher at the Migration Observatory at the University of Oxford.

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The simple answer is that we simply don't know how many unauthorized migrants there are in London or the UK in general. And the issue is that there is no data available on this population, which makes it impossible to say with any degree of precision what the actual number is or even how close all these different estimates that we've seen going around are to the truth.

[Tim Harford]

Right, so what I'm hearing is that the Thames Water Report involves educated guesses built on educated guesses, that it's based on data that's eight years old, that it suggests one in 15 Londoners are irregular migrants, not one in 12 or one in 13, and that actually many of Those people aren't irregular migrants at all. They have leave to remain, but they've been miscategorised. What a mess. Do we have any other sources of data that might let us cross check these estimates?

[Charlotte MacDonald]

A few other researchers have used the residual method to make an estimate for irregular migrants. Their numbers are lower than Pew's and they're also quite old. Now, Jonathan Porters, professor of Economics and Public Policy at King's College London, has pointed out that we do have some administrative data which we can use to sort of sense check how much of an issue it is.

[Jonathan Portes]

So when the Met Police did an exercise where they checked the migration status of everyone they arrested, they found, perhaps unsurprisingly, that foreign nationals were almost exactly as likely to be arrested as Brits. Portion of irregular migrants was absolutely tiny, down at the sort of 1% level. So that would suggest that only a relatively small proportion even of Londoners are irregular migrants. Now, of course, you might say, well, irregular migrants, for obvious reasons, try not to get arrested because it might lead to them being discovered. So there's a selection bias there. But nonetheless, the actual proportion of irregular migrants in the population might be significantly lower than suggested by some of the estimates that have been widely publicized.

[Charlotte MacDonald]

Not only that, but since Pew published its research based on data from 2017, we know there's been a big increase in legal immigration. So, all in all, coming up with a good estimate figure for a group that might not want to be counted is very difficult.

[Tim Harford]

Well, thank you, Charlotte. That's all we have time for. But please keep your questions and comments coming in to more or lessbc.co.uk. we will be back next week. Actually, twice next week, you lucky people. Thrice if you include our bite sized Saturday edition on the podcast. On Monday, I am presenting a special edition of the programme to mark the fifth anniversary of the UK going into lockdown. We've been trying to work out what we can say with any certainty about the effect that the lockdowns had on young people. What happened to education levels, jobs, mental health? Listen in on Monday to find out. The programme is broadcast at 9am on Radio 4 and on BBC Sounds. Until then, goodbye. More or Less was presented by me, Tim Harford. The producer was Charlotte MacDonald, with Nathan Gower, Josh McMinn and Lizzie McNeil. The production coordinator was Brenda Brown. The program was mixed by Rod Farker and our editor is Richard Varden.

[Charlotte MacDonald]

Best medicine Dissecting funny and fascinating medicine.

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[Tim Harford]

There's a lot going on right now. Mounting economic inequality, threats to democracy, environmental disaster. The sour stench of chaos in the air. I'm Brooke Gladstone, host of WNYC's on the Media. Want to understand the reasons and the meanings of the narratives that led us here and maybe how to head them off at the pass that's on the Media specialty. Take a listen wherever you get your podcasts.