After 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 MI5, Dame Stella Rimington. Plus: a listener requests a cost-benefit analysis of kidney donations; and Johnny Ball gives the Apprentices a maths lesson.