Response rates in case-control studies of cancer by era of fieldwork and by characteristics of study design.

Purpose: The purpose of this study was to describe time trends in response rates in case-control studies of cancer and identify study design factors that influence response rate.

Methods: We reviewed 370 case-control studies of cancer published in 12 journals during indicator years in each of the last four decades. We estimated time trends of response rates and reasons for nonresponse in each of the following types of study subjects: cases, medical source controls, and population controls. We also estimated response rates according to characteristics of study context.

Results: Median response rates among cases and population controls were between 75% and 80% in the 1970s. Between 1971 and 2010, study response rates declined by 0.31% per year for cases and 0.78% for population controls. Only a minority of studies reported reasons for nonparticipation; subject refusal was the most common reported reason. Studies conducted in North America had lower median response rates than studies conducted in Europe. In-person and telephone interviews elicited higher response rates than mail questionnaires.

Conclusions: Response rates from case-control studies of cancer have declined, and this could threaten the validity of results derived from these studies.