Princeton Election Consortium

A first draft of electoral history. Since 2004

Simple poll-based snapshot: House 230R-205D (+/-1.5), Senate 52D-48R (+/-1)

October 30th, 2010, 1:09pm by Sam Wang


[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.

Based on the easily inspected database at Pollster.com, it is possible to assign a binary outcome in each of 30 swing districts based on who’s ahead. This gives a split of 18R-12D. Added to the noncompetitive races which give 212R-193D, the total is House 230R-205D.

For the Senate, the number of races is small enough that it’s necessary to be probabilistic, at least in part. Democrats will control at least 49 seats (47 Dems + Lieberman-CT + Sanders-VT). Of the five borderline races (WA, NV, CO, IL, and WV), Democrats lead in two (WA, WV), a Republican leads in one (NV), and two are within 1 percentage point (CO, IL). Splitting the last two gives Senate 52D-48D. Note that I am not making specific predictions on these five races – my meta-analytic goal is to estimate the total number of seats, not individual outcomes.

So what’s the action item? If you’re a committed activist, I suggest you 1) give to ActBlue, 2) Get Out The Vote (GOTV) in the borderline states for Senate races (indicated in yellow), or 3) GOTV in swing House districts.

What should we make of the less-certain projections by Nate Silver, Pollster.com, and others? They give probabilities around 80% for one party or the other controlling each chamber? That’s the uncertainty they add by making their assumptions. My probability is a nominal one: conditional on the idea that pollsters are on average unbiased, what do the polls tell us? Given that assumption, which worked so well in 2004, 2006, and 2008, the prediction is clear.

For the true geeks only: What is the nominal uncertainty on these estimates? “Nominal” means: what’s the minimum uncertainty assuming that polls are all perfect samples of likely voters? Here is one argument. In the House, there are about 100 polls for 30 races. Imagine that Democrats lead in D of them. Polls give an estimate of d=D/30=0.4 and for Republicans, r=0.6. 100 trials sampling that probability should give an uncertainty in number of seats equal to 30*sqrt(d*r/100). So: 30*sqrt(0.4*0.6/100)=1.5 seats. So the 68% confidence interval is R = [228.5,231.5] seats. For the Senate the uncertainty is about 1 seat: a 68% CI of D = [51,53] seats.

Tags: Politics

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