The Predictions of Prediction Markets

On TradeSports a contract of George Bush in the winner-take-all market is currently selling for around $7. But what does this actually mean? Most traders and researchers would argue that 0.7 is the current “market probability” that the event “George Bush wins the 2004 presidential election” occurs. But this answer drives Charles Manski, an economist who recently also published an article in the September issue of Econometrica, crazy.

He says that under not-so-far-fetched assumptions, the price of a contract reveals nothing about the dispersion of traders’ beliefs and partially identifies the central tendency of beliefs. A President.GWBush2004 contract trading at 70 reveals that 70 percent of traders believe the probability of the event “George Bush wins the 2004 presidential election” to be larger than 0.7. The mean subjective probability of this event lies somewhere in the open interval (0.49, 0.91) (price/mean belief region).

No doubt the last word on this issue hasn’t been spoken but if Manski thinks he can figure out a way that market estimates are biased (for example by narrowing the set of possible belief distributions), then he should be able to come up with a market trading strategy that consistently makes money. And if he succeeds at that, he will in the process correct the market estimate.