Mark Blumenthal | January 19, 2007
Topics: Cell Phones
Yesterday, the National Center for Health Statistics (NCHS) released its latest estimates of the number of Americans living in households without landline telephones, as well as a statistic closely watched by pollsters: During the first six months of 2006, "approximately 10.5 percent of households do not have a traditional landline telephone, but do have at least one wireless telephone."
Pollsters have been watching the growth in "cell phone only" households because cell phones are largely out of reach of the traditional random digit dial sampling methods used in most conventional telephone surveys. As such, the continuing upward tend in such households illustrated by the NCHS surveys (which involve massive monthly in-person samples of Americans), should be of great interest to anyone who follows public opinion polling.
Although I have written about these issues previously (here, here, here and here), the best analysis of how this trend has affected the accuracy of public polling has been done by the Pew Research Center. Last year, in partnership with the Associated Press and America Online, they conducted parallel surveys: One using conventional telephone sampling and another that interviewed a random sample of 750 mobile phone users over their mobile phones. The study produced a report by the Pew Research Center (available in either HTML or PDF format) reached the following conclusions:
[Cell only Americans] are younger, less affluent, less likely to be married or to own their home, and more liberal on many political questions.
Yet despite these differences, the absence of this group from traditional telephone surveys has only a minimal impact on the results. Specifically, the study shows that including cell-only respondents with those interviewed from a standard landline sample, and weighting the resulting combined sample to the full U.S. public demographically, changes the overall results of the poll by no more than one percentage point on any of nine key political questions included in the study.
Of course, given the trend reported by NCHS, the cell-phone only adult population appears to be a moving target. It has more than doubled in the last two years, and the trend shows no signs of slowing. How big a problem will cell-phone only households be in 2008? Will their absence from traditional phone surveys begin to impact results? Will pollsters begin to routinely incorporate more expensive cell phone samples into their surveys? Time will tell.