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What Will We Know by 7 PM?

Andrew Gelman of Columbia University has taken a recent set of our simulations to look at what may happen conditional on the outcomes of the first states to close their polls at 6 and 7 PM. The bottom line? If those states go roughly as expected (meaning, say, an Obama win in Virginia and a close race in Indiana), we can conclude with almost literal 100 percent certainty that Obama will win the election:

Now, what if the vote margin in Virginia, Indiana, Georgia, South Carolina, and Kentucky were to equal the expected -5.7%? We pipe this assumption through our model by calculating, for each of our 10,000 simulations, the average vote margin in these five states, and then restricting our analysis to the subset of simulations for which this vote margin is within 1 percentage point of its expected value (that is, between -6.7% and -4.7%). Out of our 10,000 simulations, 2800 fall in this range; that is, we predict there is a 28% chance that McCain’s average vote margin in these five states will be between 4.7% and 6.7%. What is of more interest is what happens if this occurs. Considering this subset of simulations, Obama’s expected national popular vote margin is +4.7%, his expected electoral vote total is 343, and the conditional probability of an Obama victory is 100%: he wins the electoral college in all 2800 simulations in this condition.

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Andrew’s full paper can be found here. (And don’t forget to buy his book).

Nate Silver is the founder and editor in chief of FiveThirtyEight.

Filed under 2008 Presidential Election 23 posts

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