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Information revelation through time. Data are from the Iowa Electronic Markets for markets predicting the two-party vote shares from the 1988, 1992, 1996, and 2000 presidential elections (19). The vertical axis plots the average absolute difference between the market prediction and the actual vote share. In the week immediately before the election, the market erred by an average of 1.5 percentage points compared with an average error of 2.1 percentage points for the final Gallup poll. The longer-run forecasting performance of the market is also impressive, with an average error of only 5 percentage points 150 days before the election, a time when polls have much larger errors when interpreted as predictions. Calculations are based on data available at www. biz. uiowa. edu/iem.