Mike Mokrzycki is an independent consultant who has studied implications of the growing cell-phone-only population for survey research. He was the founding director of the Associated Press polling unit. He may be reached at email@example.com.
Sometimes a study is more intriguing not for what it finds but for what it doesn't. That's the case with the latest federal estimates, released this morning, of how many Americans can no longer be reached by landline telephones.
First, what the semiannual update from the Centers for Disease Control did find: Steadily worsening news for surveys that exclude cell phones. Americans keep abandoning landline phones at about the same pace as in the last couple years - in the first half of 2009, 21.1 percent of adults live in households with no landline, up from 18.4 percent in the second half of 2008. By a slightly different measure - particularly relevant to random digit dial surveys using households as a sampling frame - 22.7 percent of households now have only wireless phones, up 2.5 percentage points from six months earlier. (With sample sizes of 12,447 households and 23,632 adults, sampling error for overall results is generally around plus or minus 1 percentage point.)
Somewhat surprisingly, though, the CDC's National Health Interview Survey (NHIS) - the ongoing in-person study that is the benchmark for telephone status estimates in the United States - did not find a disproportionate increase in cord-cutting overall or among the poor or unemployed, despite the deep and sustained economic downturn. In the latest NHIS, 33 percent of those households falling below the U.S. Census poverty threshold were cell-only, up 2.1 percentage points from six months earlier; 14 percent of those who were unemployed or gave "something else" as their employment status (but weren't students) were cell-only, up 3 percentage points from the previous report.
"We would have expected that the recession would have led to outsized increases, both in overall rate of wireless substitution and also perhaps among the poor relative to those with higher incomes. We did not see that effect," Stephen J. Blumberg, co-author of the CDC study with Julian Luke, told me this morning.
"It appears that lifestyle issues, such as where you live, who you live with and age are still bigger predictors of cord-cutting," Blumberg said. "Ever since we've been tracking these data, income has not been a strong predictor of being wireless-only. Yes, the poor are more likely to be wireless-only than those with higher income, but that has largely reflected the fact that people who have substituted wireless for landlines are younger, more likely to still be in school, and more likely to be renters than homeowners."
The NHIS not only measures the cell-only population but attempts to gauge what proportion of Americans still have landlines but can't really be reached on them, contributing to non-coverage for survey researchers. The NHIS began tracking cellular telephone trends in 2003 to understand the implications for landline-only federal health surveys and in 2007 also started asking respondents whether in their households "all or almost all calls are received on cell phones, some are received on cell phones and some on regular phones, or very few or none are received on cell phones." Some of these results are eye-opening:
About one in seven U.S. households (14.7 percent) are "cell-mostly." Add that to the cell-only figures and at least 37 percent of households definitely or probably cannot be reached by landline. (The cell-mostly group has been growing at a slower rate than cell-only.)
Landline abandonment is most prevalent among people age 25-29, 63.5 percent of whom live in cell-only (45.8 percent) or cell-mostly (17.7 percent) households. (Not as many people age 18-24 can't be reached by landline as they're less likely than those 25-29 to live in wireless-only households, probably because some younger people still live with parents who haven't cut the cord.)
Blumberg observed: "Interestingly, we see an increase in cell-phone usage among people living with relatives, people living with children, and older adults. More people in these groups tell us they receive all or most calls on their cell phones, but they haven't given up their landlines in disproportionate numbers."
What does it all mean for surveys that only sample landline phones? Clearly, sample non-coverage is a growing problem, at least as a perception - it's easy to wonder about survey validity if more than a third of the population of interest has little to no chance of being included. True, excluding cell phones didn't appreciably harm presidential vote preference in 2008 pre-election polls. But a deep dive into a phone-status question on the 2008 national exit poll yields cause for concern for anyone interested in not just the overall horserace but understanding why different subgroups behave and think as they do - more on this in an article I wrote with two co-authors for a soon-to-be-published issue of Public Opinion Quarterly (an earlier draft, presented in May at the annual conference of the American Association for Public Opinion Research, is available here). See also extensive Pollster.com coverage of who is abandoning landlines and what it means for the survey profession.