*2.2. Study Groups*

This study used a case-control study design. All subjects had been diagnosed with chronic pulmonary diseases (ICD-9-CM 490–496) from 2010 to 2012. To confirm the accuracy of diagnosis, we enrolled patients with a minimum of two outpatient visits or one admission. The case group was defined as newly diagnosed pneumonia (ICD-9-CM 481, 482, 483, 485, and 486) from emergency or hospitalization after chronic pulmonary disease diagnosis. The first date of pneumonia was the index date. Patients with no pneumonia diagnosis from 2009 to 2013 were selected as the control group. We performed a 1:1 age and sex match to obtain index dates corresponding to the control group.
