"Efficacy and Safety of Apixaban Compared with Warfarin at Different Levels of Predicted INR Control for Stroke Prevention in Atrial Fibrillation"  
Circ May 2  
http://circ.ahajournals.org/content/early/2013/05/02/CIRCULATIONAHA.112.142158.abstract 
"Conclusions—The benefits of apixaban compared with warfarin on stroke or systemic embolism, bleeding and mortality appear similar across the range of center and patient predicted quality of INR control."