Statistical analysis
Comparisons between the constipation group and non-constipation group were using absolute standardized differences (ASD). Whenever the ASD were less than 0.1, the characteristics of the two groups were deemed to be similar. On the other hand, Kaplan-Meier analysis was applied to calculate the cumulative incidence of asthma and then log-rank test was used to test the significance. Cox proportional hazard model was applied to estimate the hazard ratio of asthma between the constipation and non-constipation groups.