With the upcoming election, I have been following my favorite prediction site: electoral-vote.com. That site has a big map showing current predictions state-by-state as well as the overall electoral vote prediction. It also shows the senate predictions. It has been amazing accurate in the past (though, of course, this doesn't mean that the sites predictions wont change considerably between now and the election). The predictions are all based on some sort of averaging of polls, and the site shows the results of each poll. What I have found interesting (and it has been noted on the site) is that some polls appear to lean toward Obama while others lean toward Romney. In other words, the polls appear to have biases.
Why? Theories abound about this, and much of it comes down to the polling methodology. The most compelling reason I have seen comes from Nate Silver's blog on the New York Times site. Silver's blog compares traditional polls, which call only land-line phones, with more modern polls, which call cell phones along with land-lines.
As shown by a chart in Silver's blog, there is a clear and consistent difference in every swing state between the two types of polls, with modern polls leaning toward Obama. This is consistent with the idea that younger people are both likely to vote for Obama and also more likely to not have landlines. This issue has been pointed out before, and a Pew Research Report in 2010 noted substantial differences in party affiliation between voters who had a landline and those who only had a cell phone.
There is no doubt that the percentage of homes without landlines is rising rapidly. See, for example, the CDC Report from last year, showing that about 30% of adults did not have a landline in 2011, about twice the percentages as 2008. This increase in wireless-only homes does not necessarily mean an increase in bias (more and more Republicans may be shedding their landlines, and thus the bias could fall even as wireless only usage increases). Still, the departure in the polls indicates that a bias persists.