A “Brute Force” Estimation of the Residency Rate for Undetermined Telephone Numbers in an RDD Survey

In random digit dial (RDD) telephone surveys, some proportion of the sampled telephone numbers cannot definitively be classified as eligible or ineligible. Each call attempt to these numbers generally results in either a busy signal or a ring with no answer. According to the profession's standard definitions, the proportion of these unresolved numbers that are, in fact, eligible is known as “e” and should be accounted for in response rate calculations. We used call records and a directory listed indicator from a survey with an extended calling period (and other features – letters,...

In random digit dial (RDD) telephone surveys, some proportion of the sampled telephone numbers cannot definitively be classified as eligible or ineligible. Each call attempt to these numbers generally results in either a busy signal or a ring with no answer. According to the profession's standard definitions, the proportion of these unresolved numbers that are, in fact, eligible is known as “e” and should be accounted for in response rate calculations. We used call records and a directory listed indicator from a survey with an extended calling period (and other features – letters, incentives, etc.) to resolve as many unknown cases as possible and thus derive an empirical estimate (.47) for “e.” We found additional support for this estimate by matching the unresolved telephone numbers from a separate survey to residency information from a commercial data vendor. The estimate of “e” applies to national RDD surveys featuring approximately a six-call design.