Non-response in a nationwide follow-up postal survey in Finland: a register-based mortality analysis of respondents and non-respondents of the Health and Social Support (HeSSup) Study

OBJECTIVE: To examine difference in mortality between postal survey non-respondents and respondents. DESIGN: A prospective cohort study with baseline survey in 1998 and comprehensive linkage to national mortality registers until 2005, the Health and Social Support study. SETTING: A population-based postal survey of the working-aged population in Finland in 1998. PARTICIPANTS: The original random sample comprised 64 797 working-aged individuals in Finland (20–24, 30–34, 40–44, 50–54 years of age; 32 059 women and 32 716 men), yielding 25 898 (40.0%) responses in the baseline postal survey in 1998. PRIMARY OUTCOME MEASURE: Registry-based primary causes of death encoded with the International Classification of Diseases (ICD-10). RESULTS: In women, HR for total mortality was 1.75 (95% CI 1.40 to 2.19) times higher among the non-respondents compared with the respondents. In men, non-response was associated with a 1.41-fold (1.21–1.65) excess risk of total mortality. Non-response associated in certain age groups with deaths due to diseases in women and with deaths due to external causes in men. The most prominent excess mortality was seen for total mortality for both genders and for mortality due to external causes among men. CONCLUSIONS: Postal surveys result in slight underestimation of illness prevalence.