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The police in the UK are reviewing their plans for protecting public events over Christmas and New Year, following the attack in Berlin. For the past two years, the terrorism threat level has been assessed as "severe", meaning an attack is thought to be highly likely. But is it possible to quantify the real risks we face, as opposed to our perception of risk? Winton professor for the Public Understanding of Risk at the University of Cambridge David Spiegelhalter puts things in perspective. (Image: Commuters. Credit: AP)