Reported participation in case-control studies: changes over time.

There is concern that participation in case-control studies has declined. To address this question, the author and colleagues reviewed data from 82 US and Canadian case-control studies published in epidemiologic journals during two periods: 1988-1990 and 1997-January 1999. The median year of data collection, which ranged from 1972 to 1996, was the primary independent variable. Reported response among both cases and controls remained constant over this time period. The regression coefficients (beta) were small: For each year, there was a change of 0.15 percentage points for cases (p = 0.56) and -0.16 percentage points for controls (p = 0.54). Possible confounders included the location where the study had been conducted and, for cases, the disease under study (cancer vs. others). After adjustment for these factors in case groups, there was still no association between year of data collection and response: For each year, there was a change of -0.20 percentage points (p = 0.43). After adjustment of results for study location among controls, there was a moderate decline over time: For each year, there was a change of -0.44 percentage points (p = 0.12). Overall, reported response in more recent studies was similar to that in earlier studies; for control groups, this may reflect changes in locations where the studies were conducted.