The maths of spies and terrorists

More or Less: Behind the Stats

In the wake of the Boston Marathon bombing and the killing of a British soldier on the streets of Woolwich in London, it emerged that the suspects were known to the security services. But how feasible is it for the authorities to keep track of everyone on their watch list? Tim Harford crunches the numbers, with the help of the former head of the UK intelligence service MI5, Dame Stella Rimington.