 The objectives of this study are, first, to identify select clinical covariates from electronic medical records EMR that might be associated with initiation of oral anticoagulant medications dabigatran or warfarin in patients with non-valvular atrial fibrillation NVAF at risk for stroke; second, to quantify the association between EMR-based clinical characteristics and patterns of insurance claims; third, to assess the potential for unmeasured confounding in dabigatran vs warfarin comparative effectiveness and safety studies based on administrative claims databases. 