The Effectiveness of Advance Letters for Cell Telephone Samples

In random digit dial (RDD) telephone surveys, advance letters mailed prior to dialing sampled telephone numbers may increase survey response rates (de Leeuw et al. 2007). The ability to mail advance letters to RDD samples relies on the availability of addresses that matched to the sampled telephone numbers. Traditionally, address matching was possible only for landline telephone samples with directory listings, which are not generally available for cell telephone numbers. It is now possible to obtain mailing addresses for a sizeable proportion of cell telephone numbers. Since cell telephone samples are now an increasingly large part of RDD telephone surveys, the use of advance letters mailed prior to dialing cell telephone numbers may result in an increase in response rates similar to those seen for landline telephone numbers. To test this possibility, mailing addresses were obtained for samples of landline and cell telephone numbers in the 2013 National Immunization Survey, a large, national, dual-frame RDD survey sponsored by the Centers for Disease Control and Prevention and fielded by NORC at the University of Chicago. Prior to dialing, advance letters were mailed to half of the cases in the landline and cell telephone samples with available addresses. In this study, we compared address match rates and address accuracy rates between the landline and cell telephone samples and measured the effect of the advance letter on survey response rates in the landline and cell telephone samples. We found that while advance letters had a positive effect on screener completion in the landline sample, they did not impact screener completion in the cell telephone sample. The lack of effect in the cell telephone sample may be due to a higher rate of inaccurate address matching than in the landline telephone sample: in the cell telephone sample, recently-updated addresses were found to be more accurate, and when the analysis was restricted to advance letters mailed to recently-updated addresses, the impact on screener completion in the cell telephone sample was similar to that in the landline sample. We also found that advance letters had a larger positive effect on interview completion in the landline sample, but sample sizes in the cell telephone sample for the experiment were too small to evaluate the impact on interview completion. Implications of these results for dual-frame RDD telephone surveys will be discussed.