Planned subgroup analyses The response to vitamin K or placebo is measured in two ways in this study. The first way is by observing the number of clinical events (thrombosis and hemorrhage) and the second is by observing changes in the INR values between the day of study drug and the next day. The number of clinical events in the study is likely to be relatively low, reducing the power of subgroup analyses to determine if baseline variables such as the degree of prolongation of the INR influence the response to oral vitamin K. However, since changes in the INR value will occur in all patients and since the INR is a continuous variable, a subgroup analysis which examines the influence of baseline variables (such as the degree of prolongation of the INR) on the response to vitamin K will have more power if performed with the change in the INR value as the dependant variable. This analysis will include: baseline demographic variables (such as gender, weight, presence of cancer, and age), the degree of prolongation of the INR, baseline maintenance warfarin dose and the indication for warfarin therapy. The ability of these variables to predict changes in the INR in both the vitamin K and placebo groups will be calculated. The ultimate goal of this line of investigations is to generate hypotheses about those factors (if any) that influence the response to vitamin K. These hypotheses could be tested in future studies within which the dose of vitamin K is modified based on the presence or absence of these risk factors.