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.
Article summary
Article focus
Women and individuals from upper social strata tend to participate more actively in postal health surveys.
What this exactly means in terms of health selection among respondents is unclear.
Postal health surveys are believed to produce underestimates of illness prevalence.
Key messages
Total mortality was consistently and for women in the age group ≥50 years and for men in the age groups ≥40 years significantly higher among non-respondents compared with respondents during a 7-year follow-up among a total Finnish nationwide sample in working age comprising almost 65 000 individuals.
The excess mortality observed was 1.5–2 fold. Among men, it was explained by external causes, whereas among women, it was due to diseases and was statistically significant only in the age group 50–54 years.
Postal surveys result in slight underestimation of illness prevalence.
Strengths and limitations of the study
The linkage to mortality data was successful for virtually all individuals of the original sample comprising nearly 65 000 individuals. The sample size secures the reliability of the conclusions drawn. Furthermore, the registry data on mortality in Finland can be considered as reliable. To the best of the authors' knowledge, a corresponding study based on an as large a sample as in this study has not previously been carried out.
Some inaccuracy concerning the final diagnosis of death is possible. A further study limitation is that data of socioeconomic status or educational level of non-respondents were not available, and hence, adjustments of the statistical analyses for these variables were not possible.