Statistical analysis
The percentage, mean (standard deviation, SD) and median (interquartile range, IQR) were used to describe the severity of AD and its population characteristic distribution. The Kruskal-Wallis rank test was conducted to examine differences in severity distributions of AD with comorbid diseases as well as seasonal factors, food-related factors, environmental factors and psychological factors. Due to regional differences in the severity of AD, data collected in this study had a two-level structure: regional level and individual level. Therefore, two-level mixed ordered logistic regression was used to evaluate each factor associated with disease aggravation and calculate the 95% confidence interval after adjusting for potential confounding factors. Statistical tests were two-tailed with P < 0.05, and data were analyzed by STATA 14.0 (StataCorp, College Station, Texas, USA).