A growing number of Americans rely solely on a cell phone for their telephone service, and many more are considering giving up their landline phones. This presents a challenge to the kind of public opinion polling done by the Pew Internet & American Life Project.

Our standard surveys rely on random, nationally-representative samples of the population of landline phone subscribes. But we have been closely following the rise of the cell-only population and have participated in some research trying to determine if cell-only users are sufficiently different from landline owners so as to raise questions about the representativeness of our landline surveys.

A new study of the issue by our colleagues at the Pew Research Center for The People & The Press finds that cell-only Americans – an estimated 7%-9% of the general public – are significantly different in many ways from those reachable on a landline. They are younger, less affluent, less likely to be married or to own their home, and more liberal on many political questions.

Yet despite these differences, the absence of this group from traditional telephone surveys has only a minimal impact on the results of surveys on politics and national issues. Specifically, the People-Press study shows that including cell-only respondents with those interviewed from a standard landline sample, and weighting the resulting combined sample to the full U.S. public demographically, changes the overall results of the poll by no more than one percentage point on any of nine key political questions included in the study.

The same pattern holds true for phone surveys that focus on technology — the kind of surveys that we do at the Pew Internet Project.

Asked in the survey about their general opinion of computers and technology, cell-only respondents are much more positive toward computers and technology than are landline-only respondents, and somewhat more positive than other cell phone users who are accessible on a landline.

But there is little difference between the cell-only respondents and cell phone users reached on a landline in their use of the internet and their access to broadband. The only significant difference in internet use is how the respondent gets service: cell-only users are less likely than others to use DSL or a dial-up line. This makes some sense because cell-only users do not have a telephone landline running into their home for their phone service, so they would have less incentive to opt for getting a DSL line that runs over the phone wires.

This research effort was undertaken by the Pew Research Center, in conjunction with the Associated Press and AOL, to assess the challenge posed by cell phones to random digit dial surveys. The project entailed a survey of 1,503 U.S. adults, with 752 interviewed in a conventional landline sample and 751 interviewed on their cell phones, using a sample drawn from a nationally representative cell telephone number database.

The interviews were conducted March 8-28, 2006 and averaged about 11 minutes in length. Among those interviewed on their cell phones, 200 (27%) said that their cell phone was their only phone. Details about the survey, including response rates, costs, and other issues, are discussed in the body of the report.

Check it out.

[Additional material added at 3:51 p.m. 5/15/2006: An Associated Press story on the research can be found here. ]