is: X Over the next year-and-a-half, just 4 of the former (0.9% per year), but 19 of the latter (4.3% per year) suffered cerebral infarction. Thus, the relative risk reduction (RRR) is (0.043 - 0.009)/0.043 = 79%, the absolute risk reduction is 0.043-0.009 = 0.034, and the number needed to be treated to prevent one stroke is 1/0.034 = 29 (or approximately 30). Applying confidence intervals to this number needed to treat, the number needed to treat could be (using the lower boundary of the confidence interval around the RRR, which was 0.52) as great as 45, or (using the upper boundary of the confidence interval around the RRR, which was 0.90) as few as 17. Now, you know that warfarin is a potentially dangerous drug, and that about 1% of patients on this treatment will suffer clinically important bleeding as a result of treatment each year [31] . Therefore, there will be one bleed in every 100 treated patients, and if the number needed to treat to prevent a stroke is 30, then for every 3 strokes prevented, one major episode of bleeding would occur. If the lower boundary of the confidence interval for the benefit of oral anticoagulants represents the truth, the NNT is 45 and for every two strokes prevented, one would cause a major bleed; if, on the other hand, the upper boundary represents the truth, the NNT is 26 and approximately 4 strokes would be prevented for every major bleeding episode. The true risk-benefit ratio