Over the last five or six years there has been increasing hand wringing about the emerging phenomenon of households that have only cell phones and no landline telephone. The problem is that those cell-only households fall outside of the standard RDD sampling frame. You sometimes see this referred to as "frame deterioration." As the number of cell only households increases so does coverage error in any RDD sample. And, of course, the phenomenon is heavily concentrated among 18-24 year olds, the age group that already is tough to get in telephone surveys. Other demographic differences amount to lower SES. When it was five percent of the population no one worried too much, but now it’s north of 10 percent and growing fast.
A number of people on the academic and government side of the business have been tracking this phenomenon and trying to gauge the seriousness of its impact. A series of studies at the National Center for Health Statistics (NCHS) have suggested that the cell only group may be different on a number of health issues including smoking and lack of health insurance, although those characteristics are also associated with younger and lower SES populations. One of the best studies to date is by Mike Brick, Sarah Dipko, Stanley Presser, Clyde Tucker, and Yangyang Yuam and was published in POQ (Vol 70:780-793) as "Nonresponse Bias in a Dual Frame Sample of Cell and Landline Numbers." The study evaluated a sample design that had both a standard landline and a cellphone sample, a somewhat cumbersome and expensive way to try to mitigate the impact of cell only households. Their conclusion: ". . . the coverage bias due to excluding cell-only households was not substantial in 2004." This finding is consistent with other studies done in about the same time period. In one particularly compelling study Scott Keeter at Pew showed that any differences in political preferences due to excluding cell-only households can be eliminated by weighting by age (POQ, Vol 70: 88-98).
So the bottom line would seem to be that we don’t need to worry yet, but as the number of cell-only households grows it likely will become more problematic. One thing for sure: there are a lot of survey methodologists focused on the problem and the papers keep coming. More on that story as it unfolds.