Inclusion Criteria:  Initiation of a NOAC or warfarin (day 0 is the initiation day) Continuous enrollment in medical and drug insurance in [-183, 0] days Diagnosis of AF in [-183, 0] days CHA2DS2-VASc score ≥2 (moderate or high risk for stroke) No prior use of NOAC or warfarin in [-183, -1] days No recent hospitalization for stroke or major bleeding in [-60, 0] days No recent nursing facility stay in [-60, 0] days  Exclusion Criteria:  Contraindication to either drug in [-183, 0] days Valvular heart disease or mechanical heart valve in [-183, 0] days Intracranial or retroperitoneal hemorrhage in [-183, 0] days Chronic kidney disease stage V, end-stage renal disease, or dialysis in [-183, 0] days Other indications for anticoagulation therapy in [-183, 0] days