Editorial The investigators reflect: what we have learned from the Deepening our Understanding of Quality Improvement in Europe (DUQuE) study In 2009, we launched Deepening our Understanding of Quality Improvement in Europe (DUQuE), an ambitious

study on the effectiveness of quality improvement systems of hospitals in eight European countries. We believe this to be the most comprehensive and detailed study of quality improve-ment systems to date in terms of the a priori development of a coherent theory to guide the measurement and analytical strat-egy, the diverse countries involved and the range of standar-dized data collected (including surveys, chart reviews, administrative data and organizational audit). The papers pub-lished in this supplement describe the key findings of the project so far [1]. Here we discuss the overarching lessons emerging from the study. Feasibility of data collection in EU healthcare settings Without the relentless efforts of healthcare professionals and patients to gather data, research on quality improvemen