Statistical analyses
The analyses were performed using the software IBM SPSS Statistics for
Windows, v26, Armonk, New York, USA. Internal missing on specific
questions in the questionnaire was low in all surveys, <5 %.
Internal missing about asthma and respiratory symptoms was treated as a
negative response while internal missing about exposures was treated as
missing. The chi-square test was used to test for differences in
proportions and the Students t-test for differences in means. Poisson
regression analysis with physician-diagnosed asthma as outcome was
utilized to estimate risk ratios (RR) with 95% CI. The models were
analyzed both unadjusted and adjusted with age, sex, allergic
rhino-conjunctivitis, family history of asthma, smoking habits and
socioeconomic status included as covariates.
The cumulative incidence of physician-diagnosed asthma was estimated
from two models of defining the population at risk. In model A,
individuals with any of the following variables at baseline were
excluded from the population at risk: physician-diagnosed asthma, ever
having had asthma, asthma medication use, recurrent wheeze or attacks of
SOB last 12 months. In model B, individuals were excluded from the
population at risk if reporting having physician-diagnosed asthma at
baseline. The cumulative incidence of asthma medication use and
recurrent wheeze was also estimated by model A and B, and additionally
by a third model C, where the population at risk excluded those
reporting asthma medication use at baseline when asthma medication use
was analyzed as the outcome variable, and where the population at risk
excluded those reporting recurrent wheeze at baseline when recurrent
wheeze was the outcome. As we did not have complete data on person-years
we used the cumulative incidence during the ten years observation period
in each cohort to estimate annual incidence rate by assuming the
incidence to be constant. The estimated annual incidence rate was
calculated as\(Incidence\ rate=\frac{a}{(10\times(b-\left(\frac{a}{2}\right))}\)where a is the number of incident cases and b the number
of subjects in the population at risk at the start of the study, as also
estimated in other studies [15,18,19].