FiveThirtyEight would make Perry an 81.8% favorite
Fri October 1, 2010 4:12 pm

Nate Silver's analysis of the Pennsylvania Senate race between Pat Toomey (R) and Joe Sestak (D) in the September 30 New York Times is applicable to the Texas governor's race. Silver's thesis is that even a small lead in the polls usually translates into solid victory in an election when all polls in a race are averaged:

Mr. Toomey’s lead is around 7 points in the polls. How have Senate and governor candidates with a 7-point lead in the polling average — with about a month to go in the campaign — fared in the past? Let’s construct about the simplest possible study around this:

Step 1. Take all polls conducted 30 to 60 days from the election.

Step 2. Average them together.

That’s it. We’re not doing any of the fancy stuff that we do in our actual Senate model, like weighting the polls based on sample size or the quality of the pollster. We’re just taking a simple average.

There is one “trick,” though: we’re only looking at races in which at least two different polling firms published a survey in the 30-to-60-day window. If you have just one company polling a race, you don’t really have much of an average, properly speaking.

[To view chart, click HERE]

Senate candidates who have a lead of 6 to 9 points in the simple polling average, with 30 days to go until the election — about where Mr. Toomey’s lead stands now — are undefeated since 1998. This isn’t quite as impressive as it sounds, since there are only seven such candidates in the database. But if we expand the scope of our study just a bit, it proves to be the norm rather than the exception. Senate candidates with a slightly larger lead in the polling average — 9 to 12 points — are also undefeated. Candidates with a slightly smaller lead in the polling average — 3 to 6 points — have a pretty good track record, with nine wins against three defeats.

I took the average of the last six polls, two of them by Rasmussen. Here are the polls:

Rasmussen July
Perry 49, White 41

Rasmussen September
Perry 48, White 32

Public Policy Polling
Perry 42, White 41

Zogby
Perry 44, White 41

Tx Newspapers (Blum & Weprin)
Perry 46, White 39

UT/Texas Tribune
Perry 39, White 33

Six poll average:
Perry, 44.66, White 39.50
Perry lead = 5.16

Perry's lead falls within the range of 3 to 6 points in which candidates with a lead in that range have won 9 out of 12 Senate races since 1998.

More pertinently, in 11 governor's races that fall into the three-to-six points range, the leading candidate has won 9 times. If you buy into this methodology, Perry is an 81.8% favorite.

[Math not guaranteed]

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