Government interventions to shore up the housing market add an extra element of unpredictability.

ECONOMIST: Bricks and slaughter

But for Mr Savall memorisation is the same as playing on autopilot, and stifles a necessary element of unpredictability.

ECONOMIST: Early music

Current and former marshals contacted by CNN took exception to Napolitano's claims, saying that although there is an element of randomness to increase unpredictability, the assignments of air marshals are largely based on intelligence and an analysis of which flights are most vulnerable or most attractive to terrorists.

CNN: Air travel security under review, officials say