Princeton Election Consortium

The near-final outcome of the 2010 elections is Senate 53D, House ~243R. What an event – the largest Republican majority in six decades. Yet a clear majority for Democrats in the Senate. This should be an interesting two years.

So how’d we do? As I wrote before, Senate poll medians did well, with Nevada as one glaring exception, leading to a 1-seat deviation from the 95% CI. There’s been talk about Hispanic voter turnout in Nevada, but I believe that’s not supported by exit polling data. It’s more likely that undecideds simply made a last-minute decision to stay with Reid.

In the House, we were off by about 13 seats, much more than the nominal error, +/-2 seats. Therefore there’s a systematic error. One possibility is bad time sampling – at the district level, polls are more spaced in time. However, we did just about as well as the famous guy, Nate Silver. In my case the tools were the Pollster database and a sheet of paper. By including national polls one could do better, not to mention other sources. Using these, a better topline (total # of seats) estimate than either of us came from Stochastic Democracy.

If you like district-level predictions, go read Stochastic Democracy. David Shor’s model that is theoretically fairly solidly based, enough to give true probabilities (for instance, House takeover 98%). His r^2 value for exact district-level vote share was much better than FiveThirtyEight. He needs to write all that up!

I have more to say about the subject of “snapshots,” “forecasts,” how they’re fundamentally the same thing, and what they’re good for. I have had interesting conversations with David (who is visiting Princeton this year) as well as correspondence with Andrew Gelman and Nate Silver. I am digesting it all, and will comment in a few days.

I will write more later, but overall the simple approach did well…though not as well as expected. Current actual outcome: Senate 52D, House about 245 243R. The House discrepancy, about 1513 seats, is equivalent to about 1.6 1.4 percentage points of popular vote. Not ideal, but pretty good. If you’re dissatisfied, consider that this transparent, low-assumption calculation did as well as assumption-laden models such as Pollster and FiveThirtyEight. Furthermore, those models only put a House takeover at ~80% probability, which was obviously wrong.

The Senate medians got the outcome of three races correct (WA/IL/WV) but not the closest race, CO. But then there’s this:

This error is so statistically glaring. I am not sure what to think about it. More discussion of this, and the House result, later.

The Gallup generic Congressional preference poll (R+15%) has drawn notice, including from ostensibly statistics-savvy poll geeks. It’s unclear why since individual data points always vary more than averages. Then again, as the saying goes, if it bleeds it leads. But there’s something more surprising. [Read more →]

Political scientist Matthew Dickinson offers a cogent essay on the difference between a real prediction and electoral “forecasting”.

In the Senate: In critical states there are so many polls that fairly exact statements can be made. The probability of the Senate remaining Democratic is nearly 100%*. Assuming polls are unbiased on average, with 85% probability the following exact outcome is predicted: 51D-49R. Knife-edge races:
WA: Murray (D) over Rossi (R) by 2.0 +/- 1.0% (mean +/- SEM).
NV: Angle (R) over Reid (D) by 2.0 +/- 0.5%.
CO: Buck (R) over Bennett (D) by 1.0 +/- 0.7% (this is the real nailbiter).
IL: Kirk (R) over Giannoulias (D) by 2.0 +/- 0.8%.
WV: Manchin (D) over Raese (R) by 4.5 +/- 1.1% (statistically a little suspenseful because only n=6 for recent stable polls).
All nominal win probabilities are over 90%.

In the House: The Republicans will take control. By the same methods as before, we’re at 230R-205D. The 95% confidence interval for a gain of 50-54 seats to 228-232R. Pollster currently has 27 “toss-up” races with a total of over 100 polls, an amazing number.

P.S. For the record, as of 10PM tonight, two model-intensive comparables are FiveThirtyEight Senate 51.7D, House 233.1R, and StochasticDemocracy Senate 52.2D, House 237.7R. Both House results are outside my 95% confidence interval. I am interested in how far my simple-minded approach can go toward top-line characterization. But who knows, maybe there’s unseen complexity.

*This goes down to 90% if there is a 50-50 split and Lieberman jumps ship again. 100% if he stays with the Democrats this time.

In terms of leverage, Colorado and West Virginia are extremely ripe for GOTV and last-minute donations. Illinois is a large state, harder to be effective; also, Kirk (R) is edging ahead. Nevada…Reid’s significantly behind Angle.

Last-minute polls will be available tomorrow. The final Senate prediction will move (indeed it already has). Also, I haven’t applied variance minimization (VM) yet for tiebreakers. In the meantime…

I should express the idea of the previous post more carefully. Let us pose a hypothesis: Pollsters sample voters with no average bias. Their errors are small enough that in large numbers, their accuracy approaches perfect sampling of real voting. This hypothesis held up well in 2004, 2006, and 2008.

The 2010 test occurs on Tuesday. Polls predict an outcome (95% confidence) of a House Republican majority of 227-233 seats. For the Senate Democrats it’s 50-52 seats (assuming Lieberman continues to caucus with them). If either outcome falls outside this range, it’s time to re-examine the hypothesis.

[Title edited to reflect the fact that polls are still changing at the last minute, so predictions won't be final until after the weekend. Notably, Reid/Angle is no longer a tossup. Reid is likely to lose.]

As I wrote in 2008, despite the fuss about fancy modeling, for topline estimates, models such as FiveThirtyEight essentially add noise – needless uncertainty. On average, pollsters know what they are doing. This can focus your efforts: donations, critical Senate GOTV (indicated in yellow), and critical House GOTV.

Historically, a simple meta-analysis of existing polls gives the most accurate predictions. In 2008, this worked beautifully, when my pre-election estimates were off by only 1 EV for President, 0 seats for House (an exact match!), and 1 seat in the Senate. So what about 2010? First off: a Republican-controlled House and a Democratic-controlled Senate are essentially certain. Here’s why. [Read more →]