In our information-crazy, never-out-of-touch world, it’s becoming harder and harder to find out who we are and what we do.

That’s the ironic truth facing epidemiologists around the country.

The popularity of cell telephones, an increasingly mobile population, rising expenses, flat budgets and new insights into ways people can answer a question differently depending on how it’s asked — all are conspiring to make health surveys more difficult.

In public health, pretty much everything depends on good data. Researchers and policymakers can’t identify a problem, figure out whether it’s serious and devise a strategy to fight it without first being able to count it. “If you can’t measure something, you won’t be able to change it” is an oft-heard aphorism.

How big a problem is obesity? Are restrictions on smoking changing people’s habits? Is autism more prevalent than it was a decade ago? Is the recession affecting people’s access to health insurance?

All are questions of national importance — and none can be answered without unbiased surveys of a representative sample of the population.

Cell telephones are perhaps the biggest threat to survey data that epidemiologists have confronted in years.

The National Center for Health Statistics reported that in the first half of last year, 16 percent of American adults lived in households that have only cell phones. This was up from 7 percent three years earlier, and rising rapidly.

The federal government’s main tool for measuring the health habits of Americans, the Behavioral Risk Factor Surveillance System (BRFSS), uses the telephone to interview a nationwide sample of adults (470,000 this year). Historically, interviewers called only conventional telephones, as all but the 2 percent to 3 percent of households with no phones at all could be reached through them. But that’s not remotely true anymore.

Surveyors, however, cannot just extrapolate from the land-line respondents. That’s because studies have shown that people who have only cell phones are different from people who don’t have them or use them only occasionally.

Young people, men and Hispanics are all more likely than the “average” American to have cell phones only. But those demographic factors don’t explain everything. Even after they are taken into account by statistical means, cell-phone-only users are different.

The BRFSS surveyors this year will include cell phone numbers in every state, with a goal of having 10 percent of the interviews done that way. But it’s easier said than done.

Federal law requires that calls to cell phones be hand-dialed; it is illegal to use automatic dialers, which are standard tools for survey and polling firms. Furthermore, a huge fraction of “owners” of cell phone numbers are children ineligible for the health surveys. Once reached, some cell phone users are reluctant to talk at length because they have to pay for incoming calls.

Consequently, it takes roughly nine calls to working cell phone numbers to get one completed survey, compared with five calls to working land-line numbers, said Scott Keeter, a polling expert at the Pew Research Center for the People and the Press, an independent opinion research group. Further, an interview conducted with someone who uses a cell phone costs 2 1/2 times as much as an interview with someone on a conventional phone. In addition to higher labor costs, most surveys now reimburse cell phone users for their minutes, either in cash or through credits to online merchants such as Amazon.com.

People’s willingness to answer questions has also been affected by the barrage of phone calls, many unsolicited, they get every day. The response rate in public opinion polls has fallen from about 60 percent two decades ago to 25 percent now, according to Keeter.

But the problem goes beyond changing technology and responsiveness. It turns out that people answer the same question differently depending on how you reach them — a “mode effect.”

For example, when a group of people with the same age, race and education are called on a conventional phone, 25 percent say they smoke, but on a cell phone, 31 percent say they do. On a land line, 38 percent say they have been tested for HIV, while on a cell phone, 54 percent say they have.

The reason for this is unclear. Ali H. Mokdad, an epidemiologist who until recently was at the federal Centers for Disease Control and Prevention and ran the BRFSS, speculates it may have something to do with the fact that people on land lines are usually at home, where they have a role and image to maintain even if they are answering in privacy.

“They are less likely to say something bad about their own behavior. It’s like ‘This is my house,’ ” he said.