2.5. Statistical analysis
The participants in the control cohort group were identified using 1:4 matching in which the nearest available person for each case was selected as a matched cohor14. We compared the baseline characteristics (age, sex, type of insurance, Charlson Comorbidity Index)15 using the McNemar test because each asthmatic patient was matched to several non-asthmatic patients. The Kaplan-Meier method was used to estimate survival curves during the follow-up period, and survival was compared among groups using the log-rank test. Hazard ratio (HRs) with 95% confidence interval (CI) for the mortality of patients in the asthma cohort compared with subjects in the control cohort were evaluated using an age- and sex-stratified Cox regression model. To evaluate the effects of asthma-related pulmonary comorbidity (e.g., COPD, bronchiectasis, pneumonia, lung cancer, and NTM infection) on mortality in patients with asthma compared with subjects without asthma, the asthma cohort was classified into two groups based on comorbidity. Next, we performed age- and sex-adjusted Cox regression to evaluate the effects of respiratory comorbidity on HR for mortality in asthmatic patients compared with subjects without asthma. All analyses were conducted using SAS 9.4 (SAS Institute, Cary, NC, USA). All tests were two-sided, and p-values <0.05 were considered statistically significant.