<qa>  How many hospitalized patients will be recruited from?AECOPD-IRS plans to enroll 7600 hospitalized patients 18 years old with main diagnosis as AECOPD. Sample size estimation is based on estimated mortality of AECOPD hospitalized patients 5. Study patients will be recruited from 40-60 hospitals across China,each hospital will recruit 152-190 patients. Hospital selection follows a multi-stage sampling strategy. 