Nine-year follow-up of a survey on smoking habits in Florence (Italy): higher mortality among non-responders

BACKGROUND: Smoking prevalence is often assessed in random samples of a population. Non-response bias has been rarely investigated. METHODS: In 1989 a survey on smoking habits in Florence, Italy, was carried out (response rate: 85%). For responders and non-responders (3,621 subjects) the life status as of 1998 was assessed. Poisson regression models were fitted to estimate age-adjusted risks of death (RR) of non-responders for overall mortality and for the most important causes of death, taking the whole series of responders, postal responders and telephone responders as the reference in different analyses. This analysis included 2,071 subjects aged >/=45 years. RESULTS: Compared to the whole series of responders, mortality from all causes was significantly higher among non-responders in males (RR = 1.74; 95% CI: 1.23-2.44) and females (RR = 2.45; 95% CI: 1.79-3.29). The higher risk was seen for smoking-related and 'other' causes of death. Among females the difference was more evident for smoking-related causes (RR = 3.14; 95% CI: 1.66-5.93), among males the higher risk was similar for both groups of causes. The excess of mortality was less evident when telephone responders alone were taken as reference. CONCLUSIONS: The follow-up of subjects enrolled in a survey on smoking habits shows high mortality risks among non-responders. The data indirectly suggest that smoking was (or had been) more widespread among non-responders, in particular among females. Therefore, the prevalence of smokers assessed through this survey, focussed on smoking habit, may be underestimated. Telephone contact with non-responders to the postal questionnaire attenuated the selection bias of responders, but even with telephone back-up the response bias persisted.