In a study with 200 participants, 100 are assigned to drug A and 100 to placebo. Among drug A group, 20 develop the outcome; among placebo group, 40 develop the outcome. What is the Number Needed to Treat (NNT)?
- A 5 ✓
- B 10
- C 20
- D 2
Correct answer: A. 5
Explanation
ARR (Absolute Risk Reduction) = Risk in control − Risk in treated = (40/100) − (20/100) = 0.40 − 0.20 = 0.20. NNT = 1/ARR = 1/0.20 = 5. This means treating 5 patients with drug A prevents one additional outcome compared to placebo. RRR (Relative Risk Reduction) = 0.20/0.40 = 50%, but NNT uses absolute, not relative, measures.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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