1. Department of Pulmonary and Critical Care MedicineWeihai Municipal Hospital Weihai Shandong 264200 P. R. China
2. Department of BiostatisticsSchool of Public HealthShandong University Jinan Shandong 250002 P. R. China
3. Department of Physical ExaminationWeihai Municipal Hospital Weihai Shandong 264200 P. R. China
4. Institute for Medical DataologyShandong University Jinan Shandong 250002 P. R. China