 This failed study emphasizes the pivotal importance of a comprehensive nomogram development phase prior to initiating a randomized controlled trial. This trial also demonstrates the need to consider other factors that affect the INR and warfarin dose: advanced age, underlying comorbidities and thrombotic disposition, abnormal liver function, inadequate or excessive vitamin K intake, altered vitamin K metabolism e.g., diarrhea or antibiotics, alcohol and drug-food interactions, and changes in warfarin preparation switching among different generics or switching to or from brand name Coumadin. The most important lesson from this cohort study is that to decrease clinical adverse events due to warfarin, it is first necessary to develop a warfarin nomogram that is superior than the current practice of educated guesses and trial and error. 