Monday, April 26, 2010


Dem Doom on Generic Ballot Still

This will be part of a series of Generic Ballot posts this week. I am currently in consultation with David Shor (a much smarter person than I) of Stochastic Democracy on an improvement to the "Bafumi" model. This post will be updated as warranted. Stay tuned.

When I first forecast a doom and gloom House of Representatives forecast for Democrats in February, I got a major response from across the spectrum. Since that time, others from my boss Mark Blumenthal to Alan Abramowitz to Nate Silver have chimed in on the conservation.

For those that do not remember, I plugged in the current generic vote estimate into a midterm House national vote model brought to us by Joseph Bafumi, Robert Erikson, and Chris Wlezien. Two months ago, the model predicted the Republicans would win the National House vote by around 8% and almost certainly win control of the House. What I love about the model is that it is the only one I know of that is not based off of the notion of "if the election were held today". Rather, the model takes into account how far out you are from a midterm election. This time adjustment gives the model an extra degree of accuracy because it turns out that the generic ballot tends to have a Democratic as well as current party of the president bias this far out from an election. That is, the Democratic party and the party of the president almost always do better on the generic ballot this far out than they do in November.

As we are now "only"
189 days until the election, I thought it was time to re-examine my initial post and see where we stand according to the model. The purpose here is not to not predict the exact percentage of the National House vote the Republicans will get OR the exact amount of House seats they will win. If I could do that, I'd be heading to Vegas. What I want to show is the chance Republicans win at least a majority of the national vote and seats in the House of Representatives.

Before we begin, let us lay down some ground rules. First, since this model was based of what I believe was entirely live interview polls (as opposed to those conducted using a computer or automated voice), I am only going to use those. That means I will not be using Public Policy Polling, Rasmussen, or YouGov/Polimetrix. This exclusion does not mean these pollsters are inaccurate (and lord knows I have defended
automated voice polls to the high heavens), but these polls may have a bias on the generic ballot that this model does not take into account. Second, I will also be taking out all Republican firm polls because I want to illustrate that the result the model outputs is not due to some Republican bias in the polling data.

With that out of the way, let us begin with a projection for the National House vote. As you can see, Republicans currently hold a 45%-42% lead in the aggregate. If you allocate undecideds based on how those who already registered a preference say they are going to vote [Republican % of vote / (Republican % of vote + Democratic % of vote)], it's about a 3.4 point Republican advantage. As the chart below illustrates and as it was in February, Republicans have not since the generic ballot was first implemented in 1946 held such a large advantage this far out from an election with a Democrat in the White House. Because of the unprecedented nature of this lead, the model projects Republicans will win the national popular vote by about 8.6%. Put another way, Democrats garner about 45.7% of the two way National House vote.

The question that should be running through your mind is how accurate is this forecast? The root mean square error is 1.82% for the Democratic share of the two way National vote. If we double the root mean square error, we will essentially (not perfectly) have your standard 95% confidence interval. So if we add 2*1.82 to our models estimate of 45.7, we end up with an upper threshold for the Democratic share of the two-way National House vote of a little less than 49.4%. Of course, 2.5% of the region outside of our 95% confidence is on the lower end (meaning actually below our estimate for the Democratic two way vote). Thus, there is only a 2.5% chance that the actual Democratic vote ends up being greater 49.4%. The model would have to be really, really off the mark for the Democrats to win the National House vote.

What about actual House seats? Each House election has a different popular vote to seats curve, which makes projecting control of the House more difficult than forecasting the National House vote. Later this year when it becomes clear how many incumbents are running for re-election and the quality of their challengers, we will be to use more precise votes to seat measurements as Bafumi et al. did in 2006 and Jonathan Kastellec, Andrew Gelman, and Jamie Chandler did in 2006 and 2008.

For now, we can use a less precise generic vote to seats estimate that Bafumi et al. supply (see
page 9). Taking the current two way generic vote and allocating undecideds as we did above, we find that the model predicts that Democrats will win only 44% (or 191) of the House seats. Now personally I do not believe a 66 seat lost for the Democrats is likely. I think it will likely be lower. Still, the standard error of the estimate of this projection is +/-3.70%. That is, we can be 68% confident that the ultimate percentage of seats controlled by the Democrats in the House will be +/-3.70 of 44%. Remember half of the 32% (100 - 68) of the uncertainty can be relegated to Democrats garnering lower than what is within our confidence interval. Only 16% of the time would we expect Democrats to do better than winning 47.70% of the House seats.

Now does mean the Democrats are dead in the water? Not automatically. Keep in mind this data is based off of only 15 elections (though the inclusion of the 2006 midterm makes no
substantive difference in the model's accuracy, see page 41). For all we know, something that really shakes up the political atmosphere may happen. But to ignore what are some pretty clear historical signs would be crazy.

As I said two months ago, I have a very hard time believing the Democrats maintain control of the House.


For the nerds out there who will actually look at the Bafumi, Erikson, and Wlezien paper, you will know that I did something slightly different than what they did. If I did use just a general average (weighting by sample size) as Bafumi et al. did and take into account not weighting by likely voters (see
page 18), I would still be projecting with the same certainty that the Democrats lose the national House vote and control of the House.

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