2.4 Statistical analysis
We used absolute standardized differences (ASD) to measure the effect
size of baseline covariates between groups. When the ASD was
<0.1, the characteristics between two groups were considered
similar. In addition, Kaplan-Meier analysis was used to calculate the
cumulative incidence of AD and a log-rank test was used to measure the
significance between groups. We used a Cox proportional hazards model to
estimate the associations of maternal antibiotic use during pregnancy
with childhood AD and associated covariates, which were reported as
crude and adjusted hazard ratios (HR and aHR, respectively) and 95%
confidence intervals. To consider the effect of infant
antibiotic/acetaminophen use on prenatal antibiotic exposure and avoid
reverse causation (AD occurred before antibiotic/acetaminophen use), we
performed an analysis with exclusion of AD diagnosed before 1 year of
age, as shown in the subgroup analysis. All calculations were performed
using SAS version 9.4 (SAS Institute, Cary, NC, USA).
Results