Number needed to treat calculation — MRCPCH TAS MCQ

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HardStatistics and evidence-based medicineNumber needed to treat calculationMRCPCH TAS

A trial of vitamin supplementation reduces fracture risk from 10% to 5% over two years in high-risk adolescents. What is the number needed to treat to prevent one fracture?

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Correct answer: A — 20

The absolute risk reduction is 10% minus 5%, which is 5% or 0.05; the number needed to treat is 1/0.05 = 20. A value of 10 would incorrectly use the baseline risk rather than the absolute risk reduction. The pearl is that NNT should be interpreted over the time horizon of the trial.

Reference: The Science of Paediatrics MRCPCH Mastercourse; CEBM statistics principles; RCPCH Theory examination syllabi