A randomised controlled trial reports a relative risk reduction (RRR) of 30% with a control event rate of 10%. What is the number needed to treat (NNT)?
- A 33 ✓
- B 10
- C 3
- D 30
Correct answer: A. 33
Explanation
Absolute Risk Reduction (ARR) = Control Event Rate × RRR = 0.10 × 0.30 = 0.03. NNT = 1/ARR = 1/0.03 = 33.3, rounded to 33. RRR alone without considering baseline risk inflates apparent benefit; NNT incorporates baseline risk and is the clinically relevant measure.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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