1 Respiratory Unit, San Paolo Hospital, Department of Scienze della Salute, Università degli Studi di Milano, Milan, Italy.
2 Department of Respiratory Disease, University of Sassari, Sassari, Italy.
3 Clinical Epidemiology and Medical Statistics Unit, Department of Biomedical Sciences, University of Sassari, Research, Medical Education and Professional Development Unit, AOU Sassari, Italy. [email protected].