Who needs RDD? Combining directory listings with cell phone exchanges for an alternative telephone sampling frame

Abstract

The traditional Random Digit Dialing method (list-assisted RDD using a frame of landline phone numbers) is clearly under threat. The difficulty and costs of completing telephone surveys have increased due to rising rates of refusal and non-contact. The completeness of coverage of list-assisted RDD samples has diminished due to the proliferation of cell-phone only households. The ability of list-assisted RDD to capture young, mobile, unmarried, and minority households is thus diminishing as well. Increasingly, survey researchers have been adding a cell phone component to their sampling frames for telephone surveys, despite the increased costs and other issues associated with RDD calling of cell phones. Recent research by Guterbock, Oldendick, and others has explored the extent to which “electronic white pages” (EWP) samples really differ from RDD samples. Oldendick et al. and Guterbock, Diop and Holian have emphasized that minority households are seriously underrepresented in EWP samples. Nevertheless, EWP samples have distinct advantages whenever a survey is aimed at a restricted geographic area. This paper considers the feasibility of combining EWP samples with cell-phone RDD, eliminating the ordinary RDD component from the sampling frame. We analyze the components of the telephone population, showing that the proposed method would fail to cover only one segment of the telephone population: unlisted landline households that have no cell phone. We analyze data from the 2006 National Health Interview Study to estimate the size of this segment, its demographic profile, the degree to which selected demographic and health behavior characteristics are different from those in the segments that this sampling strategy would capture, and what biases are present in the various sampling frames of interest. Trend data from the NHIS are used to assess how these biases are changing. A simple cost comparison is made among the RDD, RDD + cell and EWP + cell frames. The proposed alternative “EWP + cell” sampling frame provides relatively small bias compared to RDD + cell at costs comparable to RDD-only designs. The portion of the telephone universe that is excluded in the EWP + cell design is getting smaller all the time, therefore its bias relative to the RDD + cell design is decreasing over time. Overall, the EWP + cell design seems to be a useful alternative.