2008 Presidential Election Predictions

There are a number of sites out there that run simulations based on polling data to predict the result of the presidential election if it were held right now. I think the best one is at www.fivethirtyeight.com. Before I knew about this, I started playing with the polling data myself. I found a curve presented by Charles Franklin, relating polling margin to win probability. For example, a candidate ahead in a pre-election poll 55% to 45% will win around 90% of the time. Based on his curve, which I approximate with a normal cdf, I ran a simulation of 10,000 "elections" based each on the polling data on realclearpolitics.com and pollster.com. I did this a few weeks ago and Obama had an overwhelming lead (90-97% win probability). Given the recent shifts in polling trends, I decided to run the analysis again.

Although the national data is trending slightly toward McCain since the republican convention, my model still predicts Obama wins 71% of the time (realclearpolitics.com) or 90% of the time (pollster.com).

This model is incredibly basic. It does not take into account national trends, and many of the state polls lag behind McCain's recent bounce. For these and other reasons, I think these simulations over-estimate Obama's probability if the election were held today. Fivethirtyeight uses a much more sophisticated model, and has each candidate winning about 50% of the time. It will be interesting to redo this analysis over the next 55 days.

As a caveat, this analysis is likely to only be accurate immediately prior to the election. I did run this simulation using the polling data of battleground states immediately prior to the 2004 election. This simulation gave Kerry a 22% probability of winning with an average number of electoral votes of 250. He actually received 251. So although this model is about as simple as it could be, it may be a decent predictor immediately prior to the election.