A clinical trial randomises 400 patients (200 per arm) to drug vs placebo. The primary outcome (all-cause mortality) occurs in 20% of drug arm vs 30% in placebo arm. What is the Relative Risk Reduction (RRR) and Number Needed to Treat (NNT)?

• A RRR = 33%, NNT = 10 ✓
• B RRR = 50%, NNT = 5
• C RRR = 33%, NNT = 5
• D RRR = 10%, NNT = 10

Explanation

ARR = 30% − 20% = 10%. NNT = 1/ARR = 1/0.10 = 10. RRR = ARR/Risk in control = 10%/30% = 33%. Option A correctly gives RRR=33% and NNT=10. Option B errors by computing RRR as 20/40 = 50%. Options C and D have at least one incorrect value.

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

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