What is wrong with non-respondents? Alcohol-, drug- and smoking related mortality and morbidity in a 12-year follow up study of respondents and non-respondents in the Danish Health and Morbidity Survey

Aim: Response rates in health surveys have diminished over the last two decades, making it difficult to obtain reliable information on health and health-related risk factors in different population groups. This study compared cause-specific mortality and morbidity among survey respondents and different types of non-respondents to estimate alcohol-, drug- and smoking-related mortality and morbidity among non-respondents. Design: Prospective follow-up study of respondents and non-respondents in two cross-sectional health surveys. Setting: Denmark. Participants: A total sample of 39540 Danish citizens aged 16years or older. Measurements: Register-based information on cause-specific mortality and morbidity at the individual level was obtained for respondents (n=28072) and different types of non-respondents (refusals n=8954; illness/disabled n=731, uncontactable n=1593). Cox proportional hazards models were used to examine differences in alcohol-, drug- and smoking-related mortality and morbidity, respectively, in a 12-year follow-up period. Findings: Overall, non-response was associated with a significantly increased hazard ratio (HR) of 1.56 [95% confidence interval (CI)=1.36-1.78] for alcohol-related morbidity, 1.88 (95% CI=1.38-2.57) for alcohol-related mortality, 1.55 (95% CI=1.27-1.88) for drug-related morbidity, 3.04 (95% CI=1.57-5.89) for drug-related mortality and 1.15 (95% CI=1.03-1.29) for smoking-related morbidity. The hazard ratio for smoking-related mortality also tended to be higher among non-respondents compared with respondents, although no significant association was evident (HR=1.14; 95% CI=0.95-1.36). Uncontactable and ill/disabled non-respondents generally had a higher hazard ratio of alcohol-, drug- and smoking-related mortality and morbidity compared with refusal non-respondents. Conclusion: Health survey non-respondents in Denmark have an increased hazard ratio of alcohol-, drug- and smoking-related mortality and morbidity compared with respondents, which may indicate more unfavourable health behaviours among non-respondents.