What's Missing from National Landline RDD Surveys?: The Impact of the Growing Cell-Only Population

The number of cell phone only households has continued to grow - 12.8 percent of all households by the end of 2006, according to the National Health Interview Survey (NHIS). To assess the extent and nature of the potential bias in landline telephone samples created by the absence of cell-only households, the Pew Research Center conducted four independent dual frame studies in 2006, three of which included at least 200 interviews with cell-only respondents. Despite the fact that there are significant differences between cell-only and landline respondents on many important variables, across the four surveys with different substantive content we find that including a cell-only sample with a landline RDD sample produces general population estimates that are nearly identical to those from the landline sample alone. Yet, while the noncoverage problem is currently not damaging estimates for the entire population, we find evidence that it does create biased estimates on certain variables for young adults, 25 percent of whome are cell-only according to the most recent government estimate.