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. Further, the lack of wireless-only adults also plays a role in the recent, particularly eye-popping Rasmussen results in Kentucky (Rand Paul up by 25%) and Arkansas (Boozman up by 27% on Bill Halter). In 2007, state-level data showed that cell-phone only adults were particularly prevalent in Arkansas and Kentucky. While 13.6% of the nation as a whole was cell-phone only in 2007, in Arkansas that figure was 21.2% and in Kentucky it was 21.6% (PDF, page 5). Given the trends nationwide, by May 2010 it is likely that roughly one in three adults in Arkansas and Kentucky are wireless-only. This would make for an even more pronounced, localized gap than the national discrepancy found in Pew's 2010 study.
Additionally, given that New Jersey has one of the lowest rates of wireless-only adults in the country, this might also help explain why Rasmussen polls for the New Jersey Governor's campaign did not significantly differ from other polls in New Jersey.
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.