A cohort study reports that in a population with a baseline 10-year cardiovascular event risk of 15%, a statin intervention reduces the risk to 10%. The Number Needed to Treat (NNT) over 10 years is:
- A 20 ✓
- B 10
- C 15
- D 7
Correct answer: A. 20
Explanation
Absolute Risk Reduction (ARR) = 15% − 10% = 5% = 0.05. NNT = 1/ARR = 1/0.05 = 20. This means 20 patients must be treated with statins for 10 years to prevent one additional cardiovascular event. The Relative Risk Reduction = 5/15 = 33%, which sounds more impressive but fails to capture absolute benefit. NNT is the most clinically relevant measure for evaluating the practical benefit of a preventive intervention at population level.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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