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David Card, Joshua Angrist and Guido Imbens have been awarded this year's Nobel prize for economics for work that, according to their citation, "challenged conventional wisdom". The trio shared the prize for pioneering work in the use of "natural experiments", using real-life situations to work out the impact of government decisions Guido Imbens, one of the prize winners, explains the challenges of doing experiments in the social sciences, but how policy is often decided on "arbitrarty grounds". He explains the methodology behind one of their experiments regarding people's motivation to work if they are already receiving an income (in this case, lottery winnings). Photo: Overhead view of people walking in Copenhagen, Denmark Credit: Getty Images