A new study by the Pew Research Center has found that 2010 midterm election polls that excluded cellphones were biased towards Republicans.
The new data shows that landline-only interviews favored Republicans by more than 5 points.
In the polls that excluded cellphone users, Republicans were ahead by over 12 points while polls that included cellphone users had Republicans ahead by less than 8 points.
In Pew Research’s final pre-election poll in 2010, the landline sample of likely voters found Republican candidates ahead 51%-39%, a 12-point lead. In the sample that combined landline and cell phone interviews, the Republican lead was 48%-42%, a six-point advantage. The national vote for House candidates is not yet final; currently, Republicans lead by approximately a seven-point margin.
And the margin of difference between the two types of polls appears to be growing. Landline-only polls for the 2008 presidential election saw a Republican bias of only 2.4 points.
Pew found that “dual users” who had both a cellphone and a landline were missed by some landline-only surveys but the effect was mostly seen with cellphone-only users.
Dual users reached on their cell phone differ demographically and attitudinally from those reached on their landline phone. They are younger, more likely to be black or Hispanic, less likely to be college graduates, less conservative and more Democratic in their vote preference than dual users reached by landline.
Among dual users reached by landline, Republicans had a 12-point advantage among likely voters. But the GOP lead was only five points among dual users reached by cell phone. Among cell phone only voters, there was no Republican lead (a nominal 47%-44% Democratic edge).
Surveys that included cellphone users include NBC/Wall Street Journal, Pew Research Center, ABC/Washington Post and CBS/New York Times. Surveys that featured only landline users included Rasmussen, CNN/Opinion Research and Fox/Opinion Dynamics.
“The finding sends a stark warning to pollsters across the country as attention turns to the 2012 presidential election,” Dalia Sussman wrote for the New York Times.
“The National Center for Health Statistics has said that in the second half of 2009, 38 percent of 18-to-24-year-olds and 49 percent of 25-to-29-year-olds lived in households without a landline,” Bruce Drake of Politics Daily notes.