The sample size estimation was performed according to the increasing 1-year (June 2002-May 2003) incidence of COPD exacerbation in hospital at home. On this basis, al least 100 patients needed to be included in the study to allow to detect a 20% difference in the endpoint hospital iatrogenic diseases, with a statistical significance  = 0.05 and a power 1-  = 0.8 using a two-sided test.