Validation of risk stratification schemes for predicting stroke and thromboembolism in patients with atrial fibrillation: nationwide cohort study - Olesen, JB; Lip, GY; Hansen, ML; Hansen, PR; Tolstrup, JS; Lindhardsen, J; Selmer, C; Ahlehoff, O; Olsen, AM; Gislason, GH; Torp-Pedersen, C (BMJ, 2011)