It seems to me that the apparent failure of Intrade to function as a predictor/knowledge aggregator before the US presidential elections was an important natural experiment.
BTW, if you want hard numbers on this - Brier scores and RMSEs, I've calculated them for a number of predictors including Intrade. In general, Intrade was a mediocre predictor, outperformed by most predictors. In the case of the Senate races it came close to Nate Silver, but I have reason to believe that Silver scored poorly on the Senate races since Wang & Holbrook apparently called all Senate races correctly instead of missing 2 like Intrade and Silver.
Overcoming Bias: Was Intrade being manipulated in the last month?
The link is good apropos, but the question that interests me is a bit more general. It seems to me that the apparent failure of Intrade to function as a predictor/knowledge aggregator before the US presidential elections was an important natural experiment. What do you think about the explanations and implications? I don't think it's too hard to discuss the knowledge aggregation issues without being bogged down in the specificities of US politics.