Possible Harms of Theophylline in Chronic Obstructive Pulmonary Disease

Further limitations are that the patients were not selected in random fashion and there were no strict inclusion and exclusion criteria.However, this also represents a strength of the study, in that randomized controlled trials have high internal validity but only low external validity-in other words, they are an incomplete reflection of reality.Two particular aspects of the study enhance its validity: First, the patients were participating in a structured disease management program (DMP), so their management is likely to have been optimal.Second, the authors used propensity score matching.This is a statistical technique employed in observation studies to avoid potential bias and render populations comparable at the beginning of the study period.The baseline characteristics of the two groups of patients after matching show that this procedure was successful.