Roughly one third of American households rely solely on mobile phones and do not have landlines, meaning they will simply be excluded by polls that call landlines only. Potential voters who rely on cellphones belong to more Democratic-leaning demographic groups than those which don’t, and there is reasonably strong empirical evidence that the failure to include them in polls can bias the results against Democrats, even after demographic weightings are applied….
In one of the forecasts, I ran the numbers based solely on polls that do include cellphones in their samples. The vast majority of these polls also use live interviewers, since federal law prohibits automated calls to cellphones under most circumstances. (Note, however, that one or two mostly automated polling firms, like SurveyUSA, use a separate sample based on live interviewers to reach cellphone-only voters; these were included in the model run.)
In this universe, Mr. Obama seems poised for victory. The model forecasts him for a 4.1 percentage points win in the national popular vote. That compares with 2.9 percentage points in the regular FiveThirtyEight forecast, which includes polls both with and without cellphones.
Mr. Obama’s advantage is also clearer in the swing states. The cellphone-inclusive polls give him an 80 percent chance to win Virginia, a 79 percent chance in Ohio, and a 68 percent chance to win Florida, all considerably higher than in the official FiveThirtyEight forecast.
Overall, this version of the model gives Mr. Obama an 83 percent chance of winning the Electoral College, a full 10 percentage points higher than the 73 percent chance that the official FiveThirtyEight forecast gave him as of Monday night. So the methodological differences are showing up in a big way this year.