2004/2008

October 28th, 2008, by Andrew

How is the 2008 election different from 2004, beyond the (currently predicted) national swing of about 4 percentage points (enough to move from Kerry’s 49% of the vote to 53% for Obama)?

When I spoke yesterday, someone asked about the changes in the electoral map since 2004 and what did it all mean, are the red states being painted blue etc?  My quick answer is that the relative positions of the 50 states aren’t changing much, it’s the whole country that’s shifting.  This was my impression based on looking at the map and also based on generally seeing uniform partisan swing in votes and attitudes.

But what do the data say? Here’s a graph of Obama’s predicted share of the two-party vote in each state (based on Nate Silver’s recent poll aggregation) compared to Kerry’s in 2004:

I then fit a simple linear regression; here’s a map of the residuals, showing where Obama is doing particularly well or poorly, compared to last time:

I used regression residuals rather than simply plotting or map the differences because of “regression to the mean”: the predictable pattern that the Democratic vote will go down (relatively speaking) in places where they did particularly well last year, and go up where they did particularly poorly. Instead, the regression residuals show changes that are unexpected (relative to the linear model, that is; of course it’s not unexpected that McCain is doing relatively well in Arizona, but the simple linear regression of 2008 on 2004 doesn’t know about home states).

2000/2004

How big are these changes? One way to calibrate is to look at changes from 2000 to 2004. These will be close to the smallest changes we’ll ever see, since 2004 was really a replay of the 2000 election. Here’s the scatterplot:

And here’s the map of residuals:

So, the relative changes of states in 2008 seem greater than the changes in 2004. On the other hand, the 2008 estimates are based on fallible poll data; maybe the election outcomes will be less variable.

Similar Posts:

• Predicting the election outcome months ahead of time: discussion and link to revised paper with Kari Lock
• Election 2008: what really happened
• Red/blue/rich/poor: 2008 update
• Florida or Ohio? Forecasting Presidential State Outcomes Using Reverse Random Walks
• More on the swing in the House vote

Entry Filed under: Elections