With one in five Americans now using a cell phone as their exclusive phone service, it presents a challenge to people who call others for a living.

“It is still substantially more expensive to conduct interviews over cell phones,” Michael Dimock, associate director for research for the Pew Research Center for People and the Press, said. The organization calls Americans through random digit dialing to poll them on a variety of topics.

The Pew Center conducts about a quarter of its interviews over the respondent’s cell phone, Dimock said.

“We do random digit dial over landline phones and over cell phones to make sure that we’re including both,” Dimock said.

It is more expensive to call people on a cell phone because of technical and legal issues, and also the small amount of compensation sometimes given to respondents for the use of their cell phone minutes, Dimock said.

Federal Communication Commission regulations do not allow anybody to use an auto-dialer to call cell phone numbers, Dimock said. The regulation is meant to prevent marketers from calling cell phones with unsolicited calls.

However, with 20 percent of the population using just a cell phone, calling only landline phones wouldn’t result in a reliable poll.

“Pollsters have to manually dial those [cell] numbers to be compliant with the regulations,” Dimock said. “In the process of a field house with hundreds of interviewers who are dialing thousands and thousands of numbers, occasionally misdialing and having to start over, it results in a substantial increase in time” over the computer dialing the numbers, Dimock said.

The Pew Center also offers some small compensation to people who are polled on their cell phones.

“If we talk to them for 20 minutes, we’re burning up minutes they’re paying for, so we send them $5 in the mail,” he said. “That adds up if we’re doing hundreds of interviews.”

There also are methodological challenges for pollsters when considering cell phones, Dimock said. For example, polls work on the principle that everybody has an equal chance of being included in a survey, but people who have both a landline phone and a cell phone have a double chance of being called for the same poll.

Some pollsters work around that by calling cell phones, but only conducting an interview if the person answering the phone says they don’t have a landline. That also presents problems, however, because many people are rarely at home during the hours pollsters call, Dimock said.

“You might still be missing these people if you just hang up on anyone who tells you they own a landline phone,” he said.

Because Americans who use only a cell phone are overwhelmingly young, that method presents a particular problem when trying to gather the opinions of the younger generations.

“If we’re trying to analyze how young people are going to vote, if we only reach young people with landlines in their house, that’s not a random sample of young people by any stretch,” Dimock said.

However, most polling companies are developing methods, through extensive studies and surveys, to deal with the cell phone issue, he said. They recognize it’s not going away.

“This is growing at a rate that is so fast, even just two years or a year ago, it was acceptable for polls like us to not include cell phones,” he said. “It’s really over the last year to two years that you’ve seen most major polls switching to include cell phones.”

“We realize the importance of including cell phones in our surveys.”