Nate Silver expounds on a new study saying the growing number of cellphone-only adults (almost one in four) can introduce bias into political polls.
Cellphone-only households are different from their landline-using counterparts. They tend to be younger, poorer, more urban, less white, and more Internet-savvy. All of these characteristics are correlated with political viewpoints and voting behavior.
The pollsters' usual defense mechanism against this is to weight their polls by demogrpahics -- something which they need to do anyway, since polls are subject to many forms of non-response bias (for instance, it's harder to get men on the phone then women). But this is potentilly an inadequate response for several reasons. First, some characteristics that correlate with both cellphone usage and political preferences may not correspond to those that are most commonly used to weight polls. It is somewhat rare, for instance, for pollsters to weight their polls by characteristics like urban/rural location or marital status, which are predictive of both cellphone usage and political beliefs. Being cellphone-dependent also appears to be significantly correlated with media consumption habits (in particular, getting more of one's news from the Internet and less from television), which also seems to be increasingly important in determining one's political views.