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In a 3-year randomized trial, stroke occurred in 12% of patients given placebo and in 8% of patients given a new anticoagulant. How many patients must be treated for 3 years to prevent one stroke?
The absolute risk reduction (ARR) is the control event rate minus the treatment event rate: 12% - 8% = 4% (0.04). The number needed to treat (NNT) is 1/ARR = 1/0.04 = 25 patients for 3 years. The value 33 comes from mistaking the relative risk reduction (4/12 = 33%) for an NNT, and 13 comes from inverting the treatment event rate (1/0.08 = 12.5). Source: Guyatt G et al. Users' Guides to the Medical Literature, 3rd ed. (JAMA/McGraw-Hill, 2015), ch. 9 Therapy and ch. 12.2 Measures of association; Straus SE et al. Evidence-Based Medicine, 5th ed. (Elsevier, 2018).
Using the relative risk reduction instead of the absolute risk reduction when calculating NNT.
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