At a mean ( SD ) follow-up of 3.8 ( 1.7 ) years ( 2621 patient-years ) , there were 39 events among 463 patients ( 8.4 % ) in the device group for a primary event rate of 2.3 events per 100 patient-years , compared with 34 events among 244 patients ( 13.9 % ) for a primary event rate of 3.8 events per 100 patient-years with warfarin ( rate ratio , 0.60 ; 95 % credible interval , 0.41-1 .05 ) , meeting prespecified criteria for both noninferiority ( posterior probability , > 99.9 % ) and superiority ( posterior probability , 96.0 % ) .