Everything Is Obvious_ _Once You Know the Answer - Duncan J. Watts [75]
Problems like this one have led some skeptics to claim that prediction markets are not necessarily superior to other less sophisticated methods, such as opinion polls, that are harder to manipulate in practice. However, little attention has been paid to evaluating the relative performance of different methods, so nobody really knows for sure.5 To try to settle the matter, my colleagues at Yahoo! Research and I conducted a systematic comparison of several different prediction methods, where the predictions in question were the outcomes of NFL football games. To begin with, for each of the fourteen to sixteen games taking place each weekend over the course of the 2008 season, we conducted a poll in which we asked respondents to state the probability that the home team would win as well as their confidence in their prediction. We also collected similar data from the website Probability Sports, an online contest where participants can win cash prizes by predicting the outcomes of sporting events. Next, we compared the performance of these two polls with the Vegas sports betting market—one of the oldest and most popular betting markets in the world—as well as with another prediction market, TradeSports. And finally, we compared the prediction of both the markets and the polls against two simple statistical models. The first model relied only on the historical probability that home teams win—which they do 58 percent of the time—while the second model also factored in the recent win-loss records of the two teams in question. In this way, we set up a six-way comparison between different prediction methods—two statistical models, two markets, and two polls.6
Given how different these methods were, what we found was surprising: All of them performed about the same. To be fair, the two prediction markets performed a little better than the other methods, which is consistent with the theoretical argument above. But the very best performing method—the Las Vegas Market—was only about 3 percentage points more accurate than the worst-performing method, which was the model that always predicted the home team would win with 58 percent probability. All the other methods were somewhere in between. In fact, the model that also included recent win-loss records was so close to the Vegas market that if you used both methods to predict the actual point differences between the teams, the average error in their predictions would differ by less than a tenth of a point. Now, if you’re betting on the outcomes of hundreds or thousands of games, these tiny differences may still be the difference between making and losing money. At the same time, however, it’s surprising that the aggregated wisdom of thousands of market participants, who collectively devote countless hours to analyzing upcoming games for any shred of useful information, is only incrementally better than a simple statistical model that relies only on historical averages.
When we first told some prediction market researchers about this result, their reaction was that it must reflect some special feature of football. The NFL, they argued, has lots of rules like salary caps and draft picks that help to keep teams as equal as possible. And football, of course, is a game where the result can be decided by tiny random acts, like the wide receiver dragging in the quarterback’s desperate pass with his fingertips as he runs full tilt across the goal line to win the game in its closing seconds. Football games, in other words, have a lot of randomness built into them—arguably, in fact, that’s what makes them exciting. Perhaps it’s not so surprising after all, then, that all the information and analysis that is generated by the small army of football pundits who bombard fans with predictions every week is not superhelpful (although it might be surprising to the pundits). In order to be persuaded, our colleagues insisted, we