This great article from the Washington Post highlights the struggles that
some epidemiologists are facing
From the article: "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 to 3 percent of households with no
phones at all could be reached through them. But that's not remotely true
anymore."
Cell-phone only users are "different" from everyone else, creating a sample
bias if they are excluded.
Interestingly, the article notes that one reason including cell phones is
that federal law requires calls to cell phones to be hand-dialed. The fact
that people must pay for calls is also problematic for some.
http://www.washingtonpost.com/wp-dyn/content/article/2009/01/11/AR2009011102
082.html