Trends in participation rates in case-control studies of occupational risk factors 1991-2017.

OBJECTIVE: Declining participation has been observed in previous epidemiological studies, could occupational risk factor epidemiology be particularly vulnerable to this trend? The objective of this study was to assess trends of participation rates in occupational case-control studies.
METHODS: Five prominent occupational and epidemiological journals were pre-selected and all articles published between 1991 and 2017 were screened for case-control studies of occupational risk factors for chronic disease outcomes. The primary independent variable was median year of data collection, while the primary outcome variable was reported participation rate. We conducted linear regression, adjusting for study characteristics that included study gender mix, location of recruitment, disease outcome, and data collection method.
RESULTS: A total of 180 studies published in the five journals were included in the final analysis. The mean participation was higher for cases (78.9%) than for controls (71.5%). In linear regression, a significant trend of decreasing participation was observed for both cases with a percent change of -0.50 per year (95% CI -0.75 to -0.25) for cases and a percent change of -0.95 per year (95% CI -1.23 to -0.67) for controls. After adjustment for study gender mix, location, disease outcome, and data collection method, the trend remained statistically significant for both case and control groups.
CONCLUSION: Declining participation rates in case-control studies of occupational risk factors may reflect an overall decline of participation in population-based samples. Lower participation rates introduce the potential for bias and may deter future population-based studies of occupational risk factors. © Author(s) (or their employer(s)) 2020. No commercial re-use. See rights and permissions. Published by BMJ.