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Lip, Gregory Y.H.; Nieuwlaat, Robby; Pisters, Ron; Lane, Deirdre A.; Crijns, Harry J.G.M. . (2010). Refining Clinical Risk Stratification for Predicting Stroke and Thromboembolism in Atrial Fibrillation Using a Novel Risk Factor-Based Approach. Chest, 137(2), 263–272. doi:10.1378/chest.09-1584 
An interview with Sci-Hub Founder Alexandra Elbakyan,Who exactly should pay for academic research