A researcher tests whether a new screening tool reduces mortality in a population of 1000 participants. The mortality in the control group is 8% and in the intervention group is 5%. What is the Number Needed to Screen (NNS) to prevent one death?
- A 20
- B 33 ✓
- C 25
- D 40
Correct answer: B. 33
Explanation
Absolute Risk Reduction (ARR) = 8% − 5% = 3% = 0.03. NNS = 1/ARR = 1/0.03 ≈ 33.3, rounded to 33. Options A (= 1/0.05), B (= 1/0.04), and D (= 1/0.025) use incorrect denominators. NNS is the inverse of the ARR and represents how many people must be screened to prevent one outcome.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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