Mark Blumenthal | March 11, 2009
Topics: CDC , Cell Phones , NCHS
Oklahoma and DC have the highest percentage of adults living in "cell phone only" households in the nation (25.1% and 25.4% respectively) according to a new report just issued by the Centers for Disease Control (CDC). The report, issued by CDC's National Center for Health Statistics (NCHS) provides estimates of the wireless-only percentage of households and adults for all 50 states and the District of Columbia (which reports a wireless-only percentage even higher than Oklahoma). The states with the lowest number of cell-phone only adults are Delaware (4.0%), Vermont (4.6%) and Connecticut (4.8%).
Why does the CDC care about cell phone usage? As regular readers know (and as the report explains), CDC conducts a variety of critical health surveillance surveys via telephone, including the including the Behavioral Risk Factor Surveillance System, the National Immunization Survey, and the State and Local Area Integrated Telephone Survey. They need to monitor the growth of the cell-phone-only population in order to check for potential bias in their telephone surveys and guide any transition to cell phone interviewing.
Five years ago, survey methodologists at CDC started asking questions about telephone usage on the on-going National Health Interview Survey (NHIS). Since NHIS is an in-person survey with a massive sample (interviewing roughly 13,000 Americans every six months), it can provide very precise, national estimates of the cell-phone-only population. The semi-annual estimates of the cell-phone only population from NCHS have become the most anticipated numbers in survey research. Their estimates are critically important to any survey -- including most national media polls last fall -- that interviewed respondents over their cell phones in order to reach the cell-phone only population. Pollsters use the NCHS estimates to determine how to weight their combined landline, cell phone samples.
Until now, however, NCHS has not produced state level estimates of the cell phone only population because "the sample size of NHIS is insufficient for direct reliable annual estimates for most states."
How did they get around that limit? They first used the NHIS data to create statistical models (using logistic regression) that estimate the cell-phone-only percentages using demographic variables (including gender, age, race, education, household size, home ownership, employment and poverty status). They then applied their models to a much larger set of survey data collected as part of the Current Population Study conducted by the U.S. Census Bureau to create "modeled estimates" of the cell phone only population in each state (the first table in their report). The report also includes the "direct state-level estimates" (the results of the NHIS survey interviews in each state).
Like the national NCHS estimates, these new state level statistics will be closely watched -- and used -- by pollsters and survey researchers of all varieties.
PS: See also the reports today by AP's Mike Mokrzycki, The Washington Post's Jon Cohen
Jennifer Agiesta and the Wall Street Journal's Carl Bialik. Our colleague Brian Schaffner pondered the issue of cell phone only households by state last September. We have written a lot on the subject of cell-phone-only households and surveys, including a NationalJournal.com column in October.
Update: Pollster reader Joran sends in two charts he quickly created that plot the state-level NCHS estimates of the wireless only percentages of households and adults, complete with error bars (see the report for details on these not-quite-confidence intervals).