In a study, 200 patients receive drug X and 200 receive placebo. After 1 year: 40 deaths in drug X group, 60 deaths in placebo group. What is the Relative Risk Reduction (RRR) and Number Needed to Treat (NNT)?

• A RRR = 33%; NNT = 10 ✓
• B RRR = 25%; NNT = 5
• C RRR = 33%; NNT = 5
• D RRR = 20%; NNT = 10

Explanation

Event rate in placebo (CER) = 60/200 = 0.30; Event rate in drug X (EER) = 40/200 = 0.20. ARR = CER − EER = 0.30 − 0.20 = 0.10. RRR = ARR / CER = 0.10 / 0.30 = 0.333 = 33%. NNT = 1 / ARR = 1 / 0.10 = 10. RRR and NNT are complementary ways to express treatment benefit; NNT of 10 means 10 patients must be treated for 1 year to prevent one additional death.

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

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