The maths of spies and terrorists
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What is the main topic discussed in this episode?
Thank you for downloading More or Less from the BBC. This is the version of the programme first broadcast on BBC Radio 4. Here's Tim Harford.
Hello and welcome to More or Less, the programme which refreshes the numbers other programmes cannot reach. This week, this man is planning to make an unusual gift and he wants to know how much it's worth.
I think we were actually by the pool in Italy and I said, hmm, I think I want to give away my kidney.
But first, following the killing of drummer Lee Rigby in Woolwich, the security services have been criticised because, it transpires, the suspects were known to them. Surely they could have been followed around. Shouldn't the security services be paying more attention to people who are known to be dangerous? Parliament's Intelligence and Security Committee is to investigate the case. Well, I've been speaking to the former head of MI5, Dame Stella Rimmington. Is this sort of criticism, I asked, 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 so that they would be ready either to follow this person away if he went out in a car or to leap out the car and follow him if he was on foot. but that team of people would work for let's say six hours a day so you need to do it 24 hours a day 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 who is obviously receiving the information coming in from these people and directing them. Then all that information has to go to a desk officer who is in charge of the case. 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, while you're the mathematician, you do the sums. It's an awful lot of people.
I'm an economist and that sounds expensive. So 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, either because they've done something else or some source of information has reported these people.
How feasible is it to tail every suspect known to security services?
or they've been associating with other people who are already known to be bad guys. But there are degrees of potential involvement, 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. And everybody who knows anything about intelligence says there's no such thing as 100% certainty with intelligence.
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 bad guys. Or, more plausibly, imagine that the intelligence services could use computers to monitor everyone's email and phone lines and spot trouble.
So, Professor, we turn to you. Designed 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.
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Chapters
4 chapters
1
What is the main topic discussed in this episode?
0:00–3:05
2
How feasible is it to tail every suspect known to security services?
3:05–8:16
3
How many people and resources are required for 24/7 surveillance of suspects?
8:16–21:26
4
Why does automated mass surveillance produce huge numbers of false positives?
21:26–24:07