Gone are the good old days (as recently as the 2008 presidential campaign), when reaching Mr. and Mrs. John Q. Public at home on a landline could still produce a credible survey.

And failing to consistently reach cell phone-only users, who are disproportionately younger and more diverse than those reliant on landlines, is now officially producing bias in surveys, says Scott Keeter of the Pew Research Center. "The issue of capturing the opinions of cell phone users in surveys dates back to at least 2003, Keeter says, when only about 7 percent of Americans were estimated to be using cell phones exclusively.

"Now we're at three times that number," he says.

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If left unaddressed, the bias inherent in failing to reach a sufficient number of cell phone users will undermine the credibility of pollsters, says Keeter, an author of Pew's recent report, Assessing the Cell Phone Challenge.

And it will allow Americans' understanding of themselves and the world around them to be disproportionately shaped by a population that's older, whiter and more conservative.

Pew found that adults age 50 and older are "significantly overrepresented" in landline surveys, comprising 66 percent of an average sample, when, as a percentage of the overall population, they should be 40 percent.

"The cell phone-only population tends to be slightly more liberal on all kinds of issues, from support for health care reform, to presidential approval and party affiliation," Keeter says.

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