Using publicly available directories to trace survey nonresponders and calculate adjusted response rates.

In population-based surveys, sample lists are often out of date by the time data collection begins. Consequently, response rates, and the perceived validity of the survey, may be compromised by the unknowing inclusion of ineligible subjects. A strategy to address this issue is ascertainment of survey nonrespondents' eligibility status, enabling post hoc adjustment of response rates. In 1995-1996, population surveys were carried out in two Ontario, Canada, communities. Despite intensive follow-up, the status of 8949 (18.6%) of the 48218 potential subjects in these surveys remained unknown. In response, 500 "unknowns" from each community were randomly selected for tracing by using publicly available telephone directories and, where applicable, city directories. These tracing efforts classified persons into one of three groups: "ineligible" (moved before the mailing), "true nonresponder" (present when the survey was mailed), and "remains unknown" (no directory listing found). Publicly available directories clarified the status of 76.0% of potential participants, reducing the proportion of "unknowns" from 18.6% to 4.6%. Applying the estimated proportions of "ineligibles" from each area resulted in response rates adjusted from 63.8% to 71.2% and from 72.8% to 74.9% in the survey areas. Publicly available directories were used to successfully trace the majority of survey nonresponders, thus strengthening confidence in the survey's results.