Atrial Fibrillation,Aspirin,Anticoagulation Study ; Boston Area Anticoagulation Trial for Atrial Fibrillation Study ; Canadian Atrial Fibrillation Anticoagulation Study ; Stroke Prevention in Atrial Fibrillation Study ; Veterans Affairs Stroke Prevention in Nonrheumatic Atrial Fibrillation Study : Risk factors for stroke and efficacy of antithrombotic therapy in atrial fibrillation. Analysis of pooled data from five randomized controlled trials