... • The Pew Research Center looked at the three major political surveys they have conducted since June, comparing the results with cell-phone interviews included against the landline-only figures. They saw "a virtually identical pattern" on all three surveys: "In each case, including cell-phone interviews resulted in slightly more support for Obama and slightly less for McCain, a consistent difference of 2 to 3 points in the margin."
The biggest reason is a large difference they observed among registered voters under 30: Those in cell-phone-only households preferred Obama by a 35-percentage-point margin (62 percent to 27 percent), while Obama led by a much closer margin (52 percent to 39 percent) among those in households with wired phones.
• This past spring, Gallup managing editor Jeff Jones looked at six Gallup/USA Today surveys conducted during the first four months of 2008 and saw a similar pattern. Adding cell-phone interviews appeared to add roughly 2 points to Obama's total and take roughly 2 points from McCain.
I checked in with Jones this week to see if the pattern has held in the ongoing Gallup Daily tracking conducted over the course of the year. They are "still seeing the same pattern (crudely)," he wrote back, a "point or two more for McCain and a point or two less for Obama excluding cell-phone interviews." Jones hedged a bit, however, saying that they did not re-weight their wired-phone data (as Pew did) for this comparison.
• A few weeks ago, blogger Nate Silver compared the house effects of national polls that have been interviewing by cell phone with those that have not. He found that including cell phone interviews appeared to improve Obama's standing by 2 to 3 percentage points on the margin.
While each source is appropriately cautious, the pattern is consistent: Interviewing by landline phone only may cost the Obama-Biden ticket roughly 2-3 points on the margin. ...