Nonresponse in a community cohort study: predictors and consequences for exposure-disease associations

We have assessed predictors for response in a Norwegian community cohort study, with an 11-year follow-up. We also examined to what extent the association of gender, age, and smoking to the incidence of respiratory symptoms and asthma differed if the analyses were based on the 65% (n = 2,079) initial responders, or were based on the 89% (n = 2,819) who responded after three reminders. The associations between the six symptoms/asthma and the gender, age, and smoking groups amounted to 42 odds ratios. The adjusted odds ratio for responding at follow-up was 1.39 (95% CI: 1.01, 1.90) for those being middle aged at baseline compared to younger subjects. The adjusted odds ratios for responding at follow-up for those being students, unemployed, or retired at baseline were 0.50 (95% CI: 0.35, 0.73), 0.29 (95% CI: 0.16, 0.55), 0.21 (95% CI: 0.13, 0.36), respectively, compared to being employed. Of the 42 odds ratios mentioned above, 25 differed less than 10% when comparing the initial and all respondents. Twelve differed 10-20% and five differed 20-45%. The study indicates that to ensure a high participation rate in a follow-up study one should pay special attention to those being late responders, unemployed, retired, or students at baseline. No overt differences were observed in the gender, age, and, smoking associations to the respiratory disorders when the analyses were based on the initial compared to all respondents.