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Number needed to treat calculation — USMLE Step 3 MCQ

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ModeratePreventive Medicine, Epidemiology, BiostatisticsNumber needed to treat calculationUSMLE Step 3

A randomized trial comparing a new anticoagulant with warfarin reports a stroke rate of 2% in the treatment group and 4% in the control group over 2 years. Major bleeding rates are similar. Which of the following is the number needed to treat to prevent one stroke over 2 years?

Educational content. Not a substitute for clinical judgement or local policy.

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Correct answer: A — 50

Absolute risk reduction is 4% minus 2% = 2%, or 0.02. Number needed to treat is 1/0.02 = 50.

Reference: USMLE Step 3 content outline; standard biostatistics methods