Whenever physicists at the Large Hadron Collider at CERN announce their results, there is a lot of talk of sigma levels — numbers that measure their confidence in the results. But what exactly do those sigma levels mean? Why do theoretical physicists require much higher sigma levels that medical scientists? And how do sigma levels relate to so-called p-values?

David Spiegelhalter, Winton Professor for the Public Understanding of Risk, explains. (See here for a written version of this interview and here for more on the Large Hadron Collider).