DSMB: An independent Data Safety Monitoring Board (DSMB) will be constituted. The committee will consist of 2 experts who are not otherwise involved in the study. Proposed members are Dr Martin O'Donnell (Thromboembolism expert, Henderson Hospital, Hamilton, Ontario) and Dr William Geerts (Thromboembolism expert, Sunnybrook Hospital, Toronto, Ontario). The number of outcome events will be recorded for the cohort study and the randomized trial separately. The DSMB will be convened when a total of 20 (or multiples thereof) adjudication-confirmed major bleeds or thrombi have occurred in either of the randomized trial or the cohort study. While remaining masked to treatment allocation, the DSMB will review the source material describing the events. If the DSMB is concerned about the type or number of events, they will have the right to ask the steering committee to allow them to contact the study statistician and have an unblinded, unplanned, interim analysis for safety performed. Only experienced thromboembolism experts have been invited onto this committee, to ensure that such an interim analysis is not performed in response to less significant observations. If the interim analysis suggests that there is a significant safety concern (for example, if there is evidence of excess major bleeding or thrombotic events, as indicated by a P value of less than 0.05) the DSMB will have the right to recommend to the Steering Committee that the study be closed.