All analyses will be by intention to treat. The primary analysis of percentage change in time outside of therapeutic range will be by Student's t-test if the data appears normally distributed or by Mann Whitney U test if it is not. The secondary analyses will be by Student's t or Mann Whitney U (for percentage change in TTR and for maximum and minimum INR values), and by the Student's t or Fisher's exact test (for both percentages of patients with TTR > 75% & < 60%, and major warfarin related cardiovascular events). A subgroup analysis of the influence on the intervention of the numbers of days per week that high vitamin K foods are ingested will be carried out looking at an ANOVA analysis of % change in time outside of therapeutic range according to 3 possible categories for the number of days per week of consumption of high vitamin K containing foods (these being less than 2, 2 to 5, and greater than 5 days per week). In the same analysis we will also examine the effect on the intervention of the patient's global assessment of how variable their consumption of high vitamin K containing foods is. To do this we will convert the 4-point scale of possible responses into a dichotomous variable that combines the two options indicating the most variable diet, and the two options indicating the least variable diet.