The number of Americans with traditional landline telephones has declined sharply over the past three years — a trend with ramifications for phone surveys that inform policy and market research.

About one in eight households did not have a landline telephone in the first half of 2006, according to data the Centers for Disease Control and Prevention collected in its National Health Interview Survey. Three years earlier, it was about one in 20.

The percentage of adults using cell phones only was increasing 1 percentage point every six months from 2003 through 2005 but jumped 2 points in the most recent study, Stephen Blumberg, a senior scientist at the CDC, said Thursday.

Among all adults, 9.6 percent had only a cell phone in the first half of 2006, compared with 7.7 percent in the preceding six months. The overall number without landlines — 13.2 percent — includes those who have no phone at all.

The survey research industry is watching this trend closely as U.S. polling typically samples households via traditional landline telephones. That may underrepresent those most likely to only own cell phones — younger and poorer people and those who rent rather than own their home. But it’s much more costly to interview people on their cell phones.

The implications ultimately could affect government, academic, business and media surveys on politics, the economy, employment, health behaviors and many other topics.

Recent research — including an Associated Press-Pew survey last fall of cell phone users as well as landline households — indicates the cell-phone-only crowd is not yet large enough and their views not different enough to affect the accuracy of traditional polling among the general population or voters.

"This latest increase is larger than we would have predicted," said Scott Keeter, a senior researcher at the Pew Research Center. "It’s now about one in four young adults (18-24) who have cell phones only. So far, it’s not affecting our results, but it already has the potential to affect studies focused on young people, poor people and renters — groups more likely to have only a cell phone."

But the number of people without any kind of phone service is lower now, about 2 percent, than it was a decade ago, Keeter said.

Via The Denver Channel