Which pollsters are most accurate?

by Dave Kopel

Who Said You Said, Oct. 02, 2010

There's no shortage of polls these days in Colorado races. Which ones have the most credibility?

To find out, I followed the lead of my Independence Institute colleague Ben DeGrow, who blogs at Mount Virtus. Ben noted the anomaly between a recent Rasmussen poll -- which, like most other polls -- showed Republican Ken Buck ahead of U.S. Sen. Michael Bennet, D-Colo., and a poll from Greenberg Quinlan Rosner, which put Bennet ahead.

Ben looked up the track record of the two pollsters, and found that Rasmussen has historically been much more accurate than Greenberg Quinlan Rosner--at least according to highly respected polling analyst Nate Silver, who writes for the New York Times.

So, with the help of my intrepid intern Shelby Lane, we looked up Silver's rankings of some of the companies that have polled the Colorado U.S. senate or governor's race this year. The table below shows the results. The "Silver Rank" column shows the accuracy ranking of the pollster over the last 10 years, out of 261 polling companies.

Notably, Colorado's own Ciruli Associates is 4th-best in the entire nation. Ciruli's most recent Senate poll, from mid-August, showed Bennet +2. Interestingly, Ciruli also found strong support, as of August, for Proposition 101 (cut vehicle and telecommunications taxes; gradually reduce state income tax rate, but only in years when state revenue rises significantly), and a close contest for Amendment 61 (prohibit state borrowing, and allow local borrowing for up to 10 years).

The "PIE" column is "Pollster-Introduced Error." Silver explains that PIE is an estimate of "the amount of error that a pollster introduces above and beyond that which is unavoidable due to things like sampling variance. The lower a firm's PIE the better." Ciruli's PIE is 1.20 percentage points. A much worse rating belongs to the Zogby Internet polls, with a PIE of 4.17. (Silver's methodology is explained here.)

For more recent surveys in the Senate race, the companies with the strongest records are Rasmussen (20th-best out of 261) and CNN (39th best). They show Buck at +8 and +5, respectively.

Silver's rankings can help readers decide whether to take a particular poll with a grain of salt, or a whole shaker full. When reporting on the latest poll, Colorado political journalists would do readers a service by including a brief mention of the pollster's track record of reliability -- either from Silver, or from another credible analyst of polling. [See also the video above for the basics on "How to read a political poll."]

UPDATE: Just before this article was published, a new poll from the Marist Institute, based in Poughkeepsie, N.Y, showed Buck +8 among likely voters. The same poll shows the governor's race as John Hickenlooper 48, Tom Tancredo 29, and Dan Maes 19. Marist rates 24th-best out of 261 in Nate Silver's ranking of pollster accuracy

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