It's the time of the year for cough sweets, flu medication and paracetamol. But how do we know these and other medicines really work? We talked to David Spiegelhalter (Winton Professor of the Public Understanding of Risk), Sheila Bird (Professor at the Medical Research Council Biostatistics Unit), and Nigel Hawkes (journalist and director of Straight Statistics) about randomised controlled trials, why they are used and how they test if a new treatment works. You can also read an accompanying article.

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