Mercury's Blog

Tomorrow is going to be a landmark day for prediction markets. The 2008 US election cycle has been the most-polled, most-predicted, and likely the most-analyzed election in history. It’s been going for nearly two years, and I for one am glad the election will soon be over and governing (by whomever wins) will soon begin.

But why will it be a landmark day for prediction markets? Simply put: the data.

Prediction Markets and Polls – The Data

There are prediction markets on a wide variety of sites, with both play-money and real-money incentives. Iowa Electronic Markets, InTrade, and Betfair for real-money; HubDub, Inkling, NewsFutures for play-money. There are more, but these are the sites I’ve seen cited most often. (It’s too bad ConsensusPoint didn’t push TheWSX.com this election cycle.)

More importantly, there are a few sites that offer incredibly deep (and also probabilistic) analysis into polls. Most notably fivethirtyeight.com, which I seem to be checking a couple of times a day, now. There are national polls, national tracking polls, state polls, and even some state tracking polls! Fivethirtyeight in particular does deep-level statistical analysis to determine from poll results and demographic data how likely each state is to vote for each candidate.

The number of data points, from different prediction markets, polls, and poll trendlines/analysis will be immense. The sum total of data that will be available after this election should be a treasure trove for researchers, and should finally prove the accuracy of prediction markets.

But there’s a hitch…

Yes, there’s always a hitch, and it’s something I’ve discussed before. In elections, polls and prediction markets are measuring two different things.

Polls are measuring the percentage support for a candidate. Generally around 40-60% or so, unless it’s a total blow-out.

Prediction markets measure the percentage chance that the candidate will win their election. When the election is tight, around 50%, when it’s a blowout can regularly be 95%+. (Few prediction markets exist for candidates’ vote share; really only on the presidential level.)

What should you expect on Election Day?

I expect that a number of news outlets will be quoting percentages from InTrade in the run-up to the end of polls closing in the evening. Already final results contests are springing up, including one in the New York Times where you earn points based on current InTrade odds. You can also expect a LOT of volume on the markets tomorrow. But once the results start rolling in, the news is going to focus on the candidates alone. Wednesday will start the morning-after evaluation of the polls and markets, which will likely last for quite some time.

What does this mean in the end?

Comparing the results of polls and prediction markets is certainly like comparing apples and oranges. There are certainly some similarities, but they are fundamentally different.

What we need to do is evaluate how each forecasting method performed independently. For prediction markets, that means that a “failure” (where a prediction >50% didn’t happen) is quite likely a success. For polls, that means that a result just a few percentage points off (outside its MOE) is a failure.

I believe that prediction markets will come out looking quite good in this election. They’ve already proven their worth to me; when poll results might indicate a close or tightening race in places, the prediction market magnifies the difference, and in many cases demonstrates the poll volatility is just noise.

In the end, are the results from the prediction markets useful? Based on the number of times I’ve seen them cited this year… the answer is an unqualified YES. Are they perfect? No, and neither is any other forecasting system or technique.

I’m really looking forward to tomorrow…

This entry was posted on Monday, November 3rd, 2008 at 10:40 pm and is filed under General, Play-money Markets, Real-money Markets, Summary.