Statistical Analysis
Meta-analysis was conducted using Stata14.0 (StataCorp, College Station, TX, USA). Heterogeneity was assessed by χ² analysis. Data synthesis was conducted using a random-effects model if > 50%, and using a fixed-effects model otherwise. RR was converted to OR for data synthesis. Subgroup analysis was performed with stratification according to geographical location and quality grade. Meta-regression analysis was conducted using year of publication, region, incidence of asthma in the control group, sample size and quality score as explanatory covariates. Sensitivity analysis was conducted with the method of reducing one piece of literature at a time to evaluate the impact of a single study on this meta-analysis. Publication bias was examined using the funnel plot and Begg’s bias test if 10 or more studies were included. P<0.05 (2-sided) was considered to be statistically significant.