In a clinical trial, 200 patients were randomised: 100 received the new drug and 100 received placebo. Events (death) occurred in 20 drug-group patients and 40 placebo-group patients. What is the Number Needed to Treat (NNT)?

• A 2.5
• B 10
• C 20
• D 5 ✓

Explanation

Absolute Risk Reduction (ARR) = Risk in placebo – Risk in drug = 40/100 – 20/100 = 0.40 – 0.20 = 0.20. NNT = 1/ARR = 1/0.20 = 5. The Relative Risk Reduction would be 50% but NNT specifically uses the absolute difference. Options A, C, and D do not match this calculation.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

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