President Obama is peeling away. As you can see from the electoral vote (EV) estimator, he is the candidate with the momentum, not Romney. In terms of EV or the Meta-margin, he’s made up just about half the ground he ceded to Romney after Debate #1. And the indicators are still headed straight up.
A few days ago, the word was that Team Romney was buying ads in Minnesota and Pennsylvania. If he wins either of those states I will eat a bug. Ohio…a really big bug. And yes, I will post a photo.
Today I’ll address a common concern among the commentariat: will President Obama lose the popular vote? Steve Lombardo is on the case with some rather bad Excel curve fitting. It does not even show the Debate #1 bounce. Hmmm, someone take his keys away. (This is basically bad time resolution, a common problem in poll aggregation.)
Anyway, the short answer: I estimate Romney’s chance of winning the popular vote at 6%, odds of 16-1 against.
Usually, I’ve used state polls to estimate the electoral vote (EV). Others have used national polls to make claims about the popular vote (PV). Both types of polls can be used to predict both PV and EV.
In the past three elections, national-poll medians have been off from the final PV outcome by 0.3% (2008), 1.4% (2004), and 2.5% (2000). Doing some extrapolation, I estimate that for perfectly tied national polls, there is a 1 in 3 chance that one candidate will win the popular vote by more than 1.75% (i.e. standard deviation = 1.75%). This is not super-informative, but it gives a little information. I will use this to make a ”prior” expectation, i.e. we start looking at state polls with an expectation of how likely a particular margin M is, even before we have examined a single poll. Let’s call this prior P(M), where M is the popular margin.
State polls give a considerably sharper look. For example, in Obama v. McCain 2008, the Meta-Margin (defined as how much state polls have to swing to tie up the Electoral College) came very close to the actual outcome of Obama +7.3%:
We can combine the prior with the state-poll-based Meta-Margin to get a sharper estimate for next Tuesday’s PV outcome.
In the graph below, state polls give the black curve, S(M). To calculate the true likelihood L(M) of a given value of M, we calculate L=P*S. That is a Bayesian estimate – taking prior expectations into account. The figure shows P(M) in green and the final estimate L(M) in red.
On the red curve, everything to the left of zero adds up to the probability of a Romney popular-vote win: 6%.
Getting a popular vote and electoral vote mismatch can happen two ways: President Obama wins EV but not PV, or Romney does the same. These add up to 9%. This is a pretty low risk.
The exact popular-vote prediction today is Obama +2.1 +/- 1.1%. Allowing a little bit for Gary Johnson, this gives a vote share of Obama 50.3 +/-0.5%, Romney 48.2 +/-0.5%. I’ll update that to a final prediction this weekend.
If you want the MATLAB code to do your own Bayesian estimation, it’s here.
Here is the original Facebook comment thread from the temporary site.