 This study will create a computer program that can be used to help dose a drug called warfarin for the prevention of blood clotting. The study will collected specific information about those patients receiving this drug and use that information to create a computer program that will predict the effects of the drug. With this prediction program in place, the investigators can perform a series of what if I gave this amount of drug simulations to determine the best dose of drug for that patient. Once the computer programs are developed, the investigators will test the program in patients that actually need this drug. They will also include genetic information into the prediction since it has been shown that this information can affect how well the drug works. Patients will have this genetic information determined during this study. 