The maths of spies and terrorists
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Hello and welcome to More or Less on the BBC World Service. I'm Tim Harford. This week we look at the statistics behind catching terrorists. In the wake of April's Boston Marathon bombing and the killing of a British soldier, Lee Rigby, on the streets of Woolwich in London, the intelligence services have come in for heavy criticism. In both cases, we're told, the security services knew about the suspects and had carried out some investigations. In the British case, they were on some kind of terrorism watch list – Why then weren't they being followed around? Shouldn't the security services be paying more attention to people who are known to be dangerous? The UK Parliament's Intelligence and Security Committee is to investigate possible intelligence failings in the Woolwich case.
I've been speaking to the former head of the British Domestic Intelligence Service, MI5, Dame Stella Rimmington. Is this sort of criticism, I asked, really reasonable?
Well, if the implication of that criticism is that the security service should know everything that everybody who is known to them is doing every hour of every day, then it clearly isn't reasonable. I mean, I don't know how many suspects there are, but let's say, and one has read this in the newspaper, that there are about 2,000. And if you want to follow an individual around 24 hours a day, you would probably need a team of, let's say, six people. And that's probably under-egging it with cars. But that team of people would work for, let's say, six hours a day. So you'd need to do it 24 hours a day.
How did the Boston Marathon and Woolwich attacks raise questions about intelligence surveillance?
You need three shifts of that for one person. And then you might need other people, static surveillance, people sitting in a house, for example, to alert these people when the person was coming out, because you can't just sit outside the house. You've obviously got to sit somewhere else. Then you've got to have a control centre. Then all that information has to go to a desk officer, So you're building up a large group of people, and we're only talking about one of these manifold suspects we hear about, and doing that 24 hours a day, seven days a week. Well, you're the mathematician. You do the sums. It's an awful lot of people.
I'm an economist, and that sounds expensive. We're talking about tens of thousands of people, clearly. Clearly. One question that occurred to me is we sometimes talk about this list of suspects as though the bad guys are definitely on the list and there's nobody on the list by mistake.
The people who will be on the list, if there is a list, will be people who have, for one reason or another, come to the attention of the security services. But there are degrees of potential involvement involved. And the whole art of the intelligence operation is to try and identify the people who are most likely to do something and focus their attention on those people. And of course, that means that you might sometimes get it wrong.
As a matter of simple economics, then, it's not possible to follow every suspicious character around the clock. But even if we could, that would present mathematical problems of a different kind. Imagine that we had unlimited resources to spy on the bad guys. Or more plausibly, imagine that the intelligence services could use computers to monitor everyone's email and phone lines and spot trouble. Well, the internet comic and musician Professor Elemental has just such an idea.
And so, Professor, we turn to you. Design for us a machine that can detect the character of a rogue, even when he's concealed by a huge crowd of decent, law-abiding countrymen. Yours, et cetera, et cetera, sincerely and so forth, the Home Office.
Ha!
Of course it can be done. I'll simply recalibrate my needle in a haystack machine. I'll change it so it searches for ne'er-do-wells in the data ray.
Professor Elemental there from the intertubes. Since we weren't sure where to reach him by telegram, we called another professor, Howard Wehner, a professor of statistics at the Wharton School at the University of Pennsylvania in the United States.
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Chapters
4 chapters
1
What is the main topic discussed in this episode?
0:00–1:51
2
How did the Boston Marathon and Woolwich attacks raise questions about intelligence surveillance?
1:51–6:27
3
Why can't security services follow every suspect around the clock?
6:27–8:55
4
How many personnel are needed for 24/7 physical surveillance of suspects?
8:55–9:26