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New Iowa Poll May Understate Paul’s Support

I’m not always a big fan of dissecting individual polls — mostly because there are liable to be a plethora of them in Iowa and New Hampshire over the next several days and their errors will tend to be mitigated as more are added to the average.

Nevertheless, the new CNN poll of Iowa contains a methodological quirk that is worth bringing to your attention and which will probably result in the survey underestimating the support for Ron Paul.

The issue is that CNN’s Iowa poll was conducted by using a list of registered Republican voters and registered Republicans only:

Sample was drawn from the complete list of registered Republican voters provided by the Iowa Secretary of State.

What’s wrong with using a list of Republican voters for a Republican caucus poll? The answer is that it’s extremely easy for independent and Democratic voters to register or re-register as Republicans at the caucus site. Historically, a fair number of independent voters do this.

According to entrance polls in Iowa in 2008, for instance, about 15 percent of participants in the Republican caucus identified themselves as independents or Democrats on the way into the caucus site. Although the way that voters self-identify is not technically the same thing as which party they are officially registered with, this is probably a good proxy for what percentage of voters changed their registration to Republican when they signed in at the caucus location.

Most other pollsters are making some attempt to account for these voters. They are anticipating that the fraction of independents and Democrats will be at least as high as it was in 2008 if not a little higher, which would make sense since Republicans do not have a competitive Democratic caucus to compete with this year.

The recent Public Policy Polling survey, for instance, estimated that 24 percent of Iowa caucus participants are currently registered as independents or Democrats and will re-register as Republicans at the caucuses. This month’s Washington Post/ABC News poll put the fraction at 18 percent. There is room to debate what the right number is but it will certainly not be zero, as the CNN poll assumes.

Most of Mr. Paul’s slim lead in Iowa comes because of these independent voters. In the Public Policy Polling survey, Mr. Paul trailed Mr. Romney 22-20 among voters who are already registered as Republicans. However, because Mr. Paul performed very well among independents (and Democrats), he held a 24-20 lead in the poll overall. The CNN poll is quite simply missing these voters and therefore will probably underestimate Mr. Paul’s support, perhaps by several percentage points.

There is some unambiguously good news for Mr. Romney in New Hampshire, however, where the CNN poll did include independents and shows him with a very healthy-looking 27-point lead over Mr. Paul. Although the New Hampshire numbers still have the potential to change significantly both before and after the Iowa, that is a pretty healthy cushion, and Mr. Romney’s rivals appear to be losing ground at the very moment when they need to gain it. Our forecasting model now gives Mr. Romney a 90 percent chance of winning New Hampshire, up from about 75 percent previously.

Nate Silver founded and was the editor in chief of FiveThirtyEight.