Contacting controls: are we working harder for similar response rates, and does it make a difference?

Although obtaining high response rates is critical to epidemiologic studies, effort to achieve response rates is undocumented. The authors used three population-based case-control studies conducted in Utah between October 1991 and February 2003 to examine effort required for both initial contact and determination of final status. Differences in lifestyle characteristics between easy- or more-difficult-to-interview female controls were evaluated. Letter, phone, and in-person contacts were recorded to determine contact effort. Regarding effort required to achieve a final outcome, the number of contacts increased from eight to 14 over the 12-year study period. Compared with those in study A (conducted in 1991-1994), controls in studies B and C were twice as likely to require seven or more phone calls and controls in study B were twice as likely to require one or more in-person visit. Hispanic controls in study C were more likely than non-Hispanic White controls to receive an in-person visit and a noncontact letter. Compared with those more difficult to contact, those easy to contact were more likely to be overweight and less likely to have a family history of cancer. The amount of effort required to achieve similar or slightly lower response rates increased over time. This finding may in part depend on demographic characteristics of the population studied.