(Update) Don’t believe it: Rasmussen has become an outlier poll

Posted by AzBlueMeanie:

There has been a series of Rasmussen polls reported in the Arizona media in the past week. The media doesn't learn, or just doesn't care. I told you about the "house effect" GOP-bias of the outlier Rasmussen poll previously. Don't believe it: Rasmussen has become an outlier poll:

Bottom line: Arizona media is going to have to spend its own money to hire independent and reputable polling companies to do polling in Arizona to present a more accurate picture of public opinion in Arizona. If the media continues to rely on Rasmussen for press release polls, the media is presenting biased and questionable data from an outlier poll. Of course, that may very well fit the narrative that the Arizona media has decided upon for this election in the "post-truth politics" era.

Now there is additional analysis into the "house effect" of the Rasmussen poll. Open Left:: Ramussen's junk polls distort House outlook:

Rasmussen's polls have come under increasing scrutiny and criticism lately. When it comes to polling on the national congressional ballot, however, there's really nothing to discuss. Over a period of more than a year, the trend of Rasmussen's polls bears only a weak relationship to that of other pollsters.

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In short, Rasmussen's polls are not just biased by a few points in the Republican direction, as are its polls of Obama's approval rating (shown for comparison below), they reflects a strikingly different multi-month narrative, which is easily shown using Pollster.com's ability to filter pollsters.

But this information should lead any reputable news organization to refuse reporting Rasmussen polls as inherently unreliable. Open Left:: Lack of wireless-only helps explain Rasmussen outliers:

A new study from the CDC shows that 24.5% of US households, and 22.9% of US adults, were wireless-only in the second half of 2009.  This is a sharp increase over the past two years. In late 2007, 14.7% of US households, and 13.6% of US adults, were wireless-only.

This rapidly increasing level of wireless-only Americans will have an impact on political polling. Specifically, excluding wireless-only adults from political surveys will have a statistically significant, negative impact on Democratic performance in political polling. This was confirmed in a recent study by Pew, which compared the national generic ballot preference of a landline-only sample of 4,683 registered voters with a combined landline and cell-phone sample of 7,055 registered voters:

In the landline sample, Republican candidates have a 47%-to-41% margin over Democratic candidates on the 2010 generic horserace, but in the combined sample voters are evenly divided in their candidate preferences for this November (44% for each party).

There is still a margin of error in a poll with such a large sample size, but it is just barely over plus or minus 1%. As such, with an overall six-point gap, the survey shows a statistically significant difference between polls that include cell-phone only adults and polls that do not.

Previous research has shown that Rasmussen's use of a likely voter screen is not the reason why their polls now differ strongly from the trendline of all other polls. Rasmussen's likely voter polls are about six points more favorable to Republicans than other likely voter polls. Also, Rasmussen polls of all adults are six points more favorable to Republicans than other polls of all adults. This six-point pro-Republican tilt is exactly the gap found by Pew in their landline-only sample.

Since Rasmussen Reports excludes wireless-only adults from their surveys (possibly due to restrictions on automated phone calls to cell phones), it is likely that the wireless-only effect is one of the main reasons that Rasmussen surveys are now outlying compared to other polling outfits.

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The rapidly rise in wireless-only adults, along with the confirmation of the pro-Democratic tilt of those adults, helps explain some, and possibly all, of the difference between Rasmussen polls and other polling firms. Americans are dumping landlines at a rapid rate, and those Americans doing do skew heavily Democratic.

Update: As an election forecaster, I honestly don't know what to do about the emerging gap between polls that include cell-phone only and those that do not.  About all I can say is that "it's a problem." I strongly prefer to base my methodology on empirical studies of what has worked in the past when it comes to poll-based electoral forecasting, but the emergence of a very sizable, heavily-skewed cell-phone only population is a relatively new problem. Polls from 2004-2008 will not offer much guidance here.

Rasmussen rarely reports its methodology, unlike other pollsters. The Arizona media is lazy and likes the ease of press releases with topline numbers. The media never gets down into the weeds of the methodology of the poll, and with Rasmussen polls, it does not even disclose its methodology.

Once again, Arizona media is going to have to spend its own money to hire independent and reputable polling companies to do polling in Arizona to present a more accurate picture of public opinion in Arizona. If the media continues to rely on Rasmussen for press release polls, the media is presenting biased and questionable data from an outlier poll. Of course, that may very well fit the narrative that the Arizona media has decided upon for this election in the "post-truth politics" era.