Objectives When doing prospective incidence studies of e.g. asthma in the general population using questionnaires, they are often repeated with an interval of five or ten years. We commonly use questions like “Have you ever had asthma?” Reliability usually is studied as test-retest reliability assuming the same conditions. But how reliable is an affirmative response of a question of ever having had a condition after ten years?
Methods In a study of incidence and risk factors of respiratory diseases a random sample of 20000 individuals in a county in Western Sweden born between 1943 and 1973 was sent a questionnaire in 1993. It comprised items about airway symptoms and smoking. 15813 subjects answered (79%) and at follow-up in 2003 11463 of them (72%) answered the questionnaire. The proportion of a yes answer on the second occasion, conditional on having answered yes on the first occasion, with confidence intervals (95% CI) was calculated by gender and baseline age groups (20–30 years, 31–40 years, 41–50 years).
Results The proportion reporting physician-diagnosed asthma 2003 when doing so 1993 was 84.1% (418/497, 95% CI 80.6–87.2). Corresponding figures for reporting “ever had asthma” was 84.2% (534/634, 95% CI 81.2–87.0), “ever had wheeze since the age of 15” was 55.3% (1109/2005, 95% CI 53.1–57.5), ever smoking was 93.9% (5161/5496, 95% CI 93.2–94.5). The proportion reporting physician-diagnosed asthma by gender: women 85.7% (95% CI 81.1–89.6) and men 82.0% (95% CI 76.2–86.9).
Conclusion In this general population study the proportion of an affirmative response of a question of ever having had a condition after ten years, conditional on having answered yes on the first occasion, differed depending of question but not due to gender or age. Ever smoking and asthma were highly reliable in this aspect. There was no difference between reported asthma and physician-diagnosed asthma.
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