In a cohort study of 2000 individuals, an exposed group of 1000 had 80 events and an unexposed group of 1000 had 40 events. The Number Needed to Harm (NNH) is:
- A 10
- B 40
- C 25 ✓
- D 50
Correct answer: C. 25
Explanation
The risk in the exposed group = 80/1000 = 0.08 and in the unexposed = 40/1000 = 0.04. The Attributable Risk (Risk Difference) = 0.08 – 0.04 = 0.04. NNH = 1/Attributable Risk = 1/0.04 = 25. The NNH represents how many people need to be exposed to cause one additional harm.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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