Is Nate Silver's value at risk?

To find out, I spent a few hours re-building Nate Silver’s basic Monte Carlo poll simulation model from the ground up. It is a simplified version, lacking fancy pollster weights and economic assumptions and state-by-state covariance factors, but it contains the same foundation of state poll data that supports Nate Silver’s famous FiveThirtyEight model. That is, they are both built upon the same assumption that state polls, on average, are correct.

After running the simulation every day for several weeks, I noticed something odd: the winning probabilities it produced for Obama and Romney were nearly identical to those reported by FiveThirtyEight. Day after day, night after night. For example, based on the polls included in RealClearPolitics’ various state averages as of Tuesday night, the Sean Davis model suggested that Obama had a 73.0% chance of winning the Electoral College. In contrast, Silver’s FiveThirtyEight model as of Tuesday night forecast that Obama had a 77.4% chance of winning the Electoral College…

When state polls are accurate, FiveThirtyEight looks amazing. But when state polls are incorrect, FiveThirtyEight does quite poorly. That’s why my very simple model and Silver’s very fancy model produce remarkably similar results — they rely on the same data. Garbage in, garbage out.