The incidence and prevalence analyses from the Finnish Twin Cohort

The study population was based on the Finnish Twin Cohort, which consists primarily of adult twin pairs with both members born before 1958 and alive in 1967. Pairs of individuals with the same birth date and sex, as well as the same surname and local community at birth, were identified from the Central Population Registry of Finland. These selection criteria also captured a small number of unrelated individuals. A postal questionnaire was sent to cohort members in the autumn of 1975 (response rate, 89%), with questions on twinship, medical news Canada history, symptoms, state of health, social factors, and psychological traits. Twinship was determined from questionnaire responses and confirmed, when necessary, from birth records in local parish registers. Follow-up postal questionnaires were sent in 1981 and 1990, with response rates of 84% and 77%. Each questionnaire asked whether asthma, allergic rhinitis including hay fever (called hay fever from here on), or chronic bronchitis had ever been diagnosed by a physician. The question was “Have you ever been told by a doctor that you have or have had asthma/allergic cold, eg, hay fever/chronic bronchitis?” This question was identical in all three questionnaires.

The incidence and prevalence analyses were based on different sets of subjects selected from the Finnish Twin Cohort. The study population for incidence analysis consisted of all twins who answered all three questionnaires, whereas prevalence analyses were based on all subjects responding to the questionnaire of the year for which prevalence was calculated, ie, 1975, 1981, or 1990. The incidence analyses respondents were born from 1930 to 1957 (N = 11,540), and 45% were men. Both twins of 3,753 pairs responded in addition to 4,034 twins whose twin sibling did not respond. Prevalences, by contrast, were calculated among subjects aged 30 to 59 years at the time of each questionnaire, except for the 1990 study, when the range was 33 to 59 years. The numbers of subjects were 14,468 in 1975, 15,317 in 1981, and 12,219 in 1990. The data for calculating 1975 prevalences included some singletons (N = 2,085, 14.4% of all subjects), whereas 1981 and 1990 prevalences were based on twins only. All prevalences and incidences were calculated from individuals.