Procedure for dealing with patients who are eligible for the study on more than one occasion: Patients can only be enrolled in either the randomized trial or the cohort study once. However, a patient previously enrolled in the cohort study who presented on a later occasion with an INR of 4.5 to 10 would be eligible for the randomized trial. Similarly, patients previously enrolled in the randomized trial are eligible for the cohort study. To ensure that the results of the randomized trial are not confounded by participation in the cohort study, a minimum of one month must pass between the time a patient is enrolled in the cohort study and their subsequent enrollment in the randomized clinical trial.