Science Magazinewww.sciencemag.org SCIENCE VOL 320 16 MAY 2008 877 POLICYFORUM P rediction markets are forums for trad- ing contracts that yield payments based on the outcome of uncertain events. There is mounting evidence that such markets can help to produce forecasts of event out- comes with a lower prediction error than con- ventional forecasting methods. For example, prediction market prices can be used to
